Just to add another element to Cascabel's excellent answer, have a look at the table on page 16 of this source. (It's from the same website that one of Cascabel's sources comes from, which is a great resource for food safety information in general, with documents mostly written by an expert with numerous citations to the food safety literature.)
Anyhow, that table shows safe holding times at various temperatures based on the following assumptions:
These times are derived from the growth of pathogenic microorganisms
in food. They are based on the cold holding standard established in
the FDA Food Code that food at 41°F can be held for 7 days. These
times at specified temperatures are based on the assumption that the
food is of average quality when obtained from the food market or
supplier.
The table provides the following data points for the length of safe food preservation:
- 29F (-1.7C) or lower - "Safe" (indefinitely - no pathogenic bacterial growth)
- 30F (-1.1C) - 123.8 days
- 35F (1.7C) - 19.3 days
- 40F (4.4C) - 7.5 days
- 41F (5.0C) - 6.5 days
- 45F (7.2C) - 4.0 days
- 50F (10.0C) - 2.4 days
The table continues upward, eventually hitting the minimum "safe" time of around 4 hours at 110-115F, which was the rationale for the old "4-hour rule" that specified maximum time food could be in the "Danger Zone." (The newer 2-hour guideline seems to take into account a wider margin of error, including possible misunderstandings of the rule, improper storage, transportation, handling during prep time, etc.)
In any case, the important thing to take away is that the "Danger Zone" is not some monolithic entity, and its boundaries are a bit fuzzy. There's a widely-held belief that bacteria immediately begin growing rapidly when you hit the low point of the "Danger Zone," but it's not true. Bacteria growth happens quite slowly at cool temperatures. And as Cascabel notes, some bacteria will still grow below the "Danger Zone" limits as defined by most countries. Thus, 4C/5C/8C or whatever are not some magical limit on bacteria growth -- they are instead a practical guideline based on some safe holding time assumptions.
As mentioned above, the FDA's assumption seems to be based on 7 days of safe holding. The standards mentioned in the question which have 7-8C are probably targeted at around 3-4 days of holding time. Given that Europeans tend to shop more frequently, have smaller refrigerators, and store perishable food in them for shorter times than typical Americans, the difference in the guidelines doesn't surprise me at all.
Also, a final important point is that not all spoilage bacteria are equally dangerous. Some cause serious illness, some will cause mild gastrointestinal discomfort, and some are essentially benign to consume but will cause the food to taste (or smell or look) awful. At different temperatures some types of bacteria will outcompete others in growth. At higher temperatures, it's clear that pathogenic bacteria can grow quickly and cause illness when the food is consumed. At cool and cold temperatures, other spoilage bacteria often grow faster than the pathogens.
So, it's not just enough to say that bacteria X can multiply above 4C or 8C or whatever. You need to take into account whether bacteria X is likely to grow fast enough to accumulate enough concentration of bacteria and/or toxins to cause illness before some other non-dangerous spoilage bacteria/yeast/mold grows and makes the food unpalatable enough that people will just throw it out. (Note that some spoilage bacteria can grow at even lower temperatures, down to 23F/-5C or so, but these won't cause foodborne illness, just spoilage.)
If you read other documents on the site linked above, you'll find some references to scientific literature suggesting that much of the time food up to somewhere around 55-60F (around 15C) "spoils safe." In other words, at low temperatures, even if pathogenic bacteria grow, in many cases the random not-so-dangerous spoilage microorganisms grow faster and will spoil the food (make it unpalatable) before it becomes dangerous to eat. (The site goes so far as to claim the FDA's recommendations are incomplete in their reasoning, saying a temperature threshold of 50F (10C) for holding fresh food should be sufficient to promote safety according to HACCP science. I personally wouldn't change my fridge temperature based on that, but it's an interesting conclusion given the inconsistent guidelines from the question. Also, note this guideline is only for fresh foods; elsewhere the site recommends a maximum holding temperature of 38F for cooked leftover foods to guarantee "safe spoilage.")
Epidemiological evidence concurs with this assessment: unless the food is highly contaminated to begin with, there are few outbreaks that can be traced to food which was always kept quite cool. On the other hand, if food is kept cool but is stored above refrigerator temperatures, it is growing bacteria, and thus the higher concentration of bacteria will have a "head start" and will be more likely to grow to dangerous levels if subsequently cooked slowly, handled poorly during prep, etc. -- this is the likely the real reason behind the "Danger Zone" lower bound.
The takeaway message here should be that microorganism growth rates, times, and temperatures are quite complicated, and national guidelines are designed to be simple and easy to follow. But an oversimplified guideline has to be based on complex assumptions that come from various elements of microbiology, likely resulting in slightly different temperature standards.
