Almost universally the temperature tolerance (and any other environmental tolerances) vendors will print on packaging, labels and in manuals should be considered the guarantees for their product.
They will rarely represent the actual temperature limits a product can sustain.
That 220 °C should be read as:
the product can be heated to 220 °C repeatedly and for long times without adverse effect and/or negatively impacting the usable life time expectancy of the product and/or the safety of the user.
That doesn't mean: "the product will immediately discolour, fail, melt, warp or otherwise become damaged, unusable or unsafe when exposed or heated to temperatures of 221 °C or beyond."
Such deterioration should not occur until the product gets exposed to much higher temperatures.
There is probably no single exact number that can be considered as "safe until here, above this is immediately too high" as the rate of deterioration will depend both on the actual excessive temperature as well as the duration of exposure, in other words, there's some bandwidth. As Wikipedia describes the safety of teflon: :
... [PTFE is] stable and nontoxic at lower temperatures, it begins to deteriorate at temperatures of about 260 °C (500 °F), it decomposes above 350 °C (662 °F), and pyrolysis occurs at temperatures above 400 °C (752 °F)
An animal study conducted in 1955 concluded that it is unlikely that these products would be generated in amounts significant to health at temperatures below 250 °C (482 °F). ...
So there is some safety margin in that 220 °C.
As to why 220 °C and not another maximum temperature gets guaranteed we can only guess:
Joe's answer is one possible explanation
a different consideration is that setting a temperature limit is much more neutral than phrasing usage restrictions as prohibitions.
From a marketing perspective you avoid actually stating outright that the product is NOT suitable for something i.e. "not suitable for use on open fire" or "not suitable for specific tasks in a professional kitchen" (when you consider for examples the temperatures that a salamander can reach and maintain).
That would suggest inferior quality and drive away potential customers. Also an exhaustive list of usages considered not safe might imply everything omitted there is indeed safe.