> it is hard to compare figures from 2 different systems which don't categorize in the same way.

That is correct, but if the source has any credibility at all, we can reasonably conclude that the rates of violent crime are similar enough to discredit any assertion that Americans are especially predisposed to violence.

At least in comparison with the UK.

We don't need to know the precise degree of equivalence to draw so general a conclusion. (Which, perhaps coincidentally, accords well with common sense.)