"Safe" is not a property of the natural world waiting to be discovered. It's a matter of standardization. A regulatory body determines criteria to be applied such that a food item can be pronounced safe.
We can't know why the two thermometers' designers have decided to show different temperatures. Maybe they used two different safety standards, either created by two different regulatory agencies, or two different versions of the same standard (standards get changed with time). Note that TFA Dostmann is a German company and as far as I know, the BfR (which should be the relevant agency in Germany) has not published a temperature table for meat preparation. So it's possible that the thermometer designers didn't refer to an official standard, but tried to make rhyme of whatever safety information they were able to come across.
Another explanation is that the thermometers' display may be geared not towards safety, but towards quality. People have different preferences when it comes to what is the best taste, so for some, meat roasted to 73 internal is better than when roasted to 75, and for others it's the other way around.