The US has a lower murder rate than all but a handful of countries in the Western Hemisphere, which would be:
Martinique, Canada, Chile, and Argentina.
Last I checked these countries had lower murder rates per capita than the US and also fall within the Western Hemisphere
Parts are France are also with the Western Hemisphere Algeria also has parts within the Western Hemisphere
So the list is longer than you make it out to be.
In political discussions, the Western Hemisphere almost always excludes old world countries. Sometimes it's even defined to start at 20 longitude for clarity, though it really should be smaller than a half-sphere for that to work out right.
But if you do include those countries, the US would rank about 12th lowest out of around 40 countries.
And out of the countires that are considered the developed world the US ranks at number 1 in murder in rate.
Depending on which list you go by, I don't think we're ever at the top. Which list are you using? OECD?
In geographical terms any places west of the Prime Meridian, up to the International Date line are in the Western Hemisphere.
Now the wikipedia article does say in political rhoteric it is used to apply to the Americas,
Lets look at one of the more commonly used definitions of Rhoteric
1.(in writing or speech) the undue use of exaggeration or display; bombast.