You need to be sure you're actually comparing the same numbers. Most "violent crime" numbers I've seen from the US only include aggravated assaults or worse, where other nations include a wider range, which necessarily increases the per capita.
If you provide links to the two numbers you're basing that on, I could probably point out the shenanigans. Using the
FBI's data for the US for 2010, for instance, the rate per 100,000 citizens was 403.6. This includes murder, nonnegligent manslaughter, forcible rape, robbery, and aggravated assault. "Aggravated assault" requires the use of a weapon and/or serious bodily harm; so punching a guy out wouldn't qualify as "violent crime" according to the US statistics.
The numbers people are quoting from the UK include
all types of assaults, not just the aggravated assaults. Half of all the "violent crimes" recorded in the lastest 2009/2010 data, for instance (49%, to be precise) involve
no injury at all (
Source, the Home Office's report, bottom of page 50)
In short; they're two completely different numbers, and you can't say "The UK is like 4-5 times as violent as the US" by using them. Not if you
actually read the studies, rather than just snagged the numbers flagged as "violent crime" without reading what each study
meant by "violent crime".
For random people posting on forums, that's probably just a case of you not reading the reports themselves. In the case of the reporters using those numbers in this manner, though, it's explicitly a deliberate attempt to mislead their audience as to the reality of the situation, to further the goals of those who fund them or their own political goals. They know better, or at least, they
should.