Well this is an argument that I've had many times with people in the gamerscore/cheevos community. Not everyone is on my side on this and it's a bit of a controversy.
My Response on a thread iis:
"Yes, I understand how that can be confusing and the logical meaning for Cheevos would be the number of Achievements you have, but this is slang.
Cheevos is a street word which has no rules, rumored to first be an internal word used in Microsoft.
It doesn't have to be logic, it just is because that's the way people use it. When I say I have over 50k cheevos, you know what I'm talking about.
When I say I have 50,000 xbox gamerscore, it doesn't flow right. 1st because it's a lot longer to type xbox gamerscore than cheevos. 2nd gamerscore is a plural with out an s at the end. It sounds so much better when you're talking up your epeen when you have an s at then end of the word you're talking about.
It's just the way it is and the way people use it."
If you're interested in a 31 page thread about this topic you can go to the link below. But save yourself some time and just believe me on this one. Just go to the urban dictionary.