The nuance here is of cause that there are a bunch of people using multiple browsers.
Also I mean there are a lot of people using browsers on the world
If 10% of firefox users are also iOS users, which is not unlikely, then those people get double-counted. In my case I probably use my phone and tablet for at least 50% of my web traffic, not counting youtube, which also skews things.
Granted, they're probably just as accurate as netcraft. /shrug