I don't find that particularly hard to believe. There's easily 20x more firearms per capita in the US than the UK. US police are routinely armed, while UK armed police are only deployed where a firearm is believed to be involved. The abundance of firearms in the US means that police need to be ready for a potential firearms incident in every encounter with the public.

Of course, the number of unjustified deaths will likely be disproportionately higher simply because all US police have the ability to react to a situation with lethal force. Sadly, as long as guns are as abundant as they are in the US, you'll never reduce that to the same level as the UK.