I thought this was an interesting question, and since Bell's is a brewery that is close to me and because I've liked their beers a lot, I thought I'd take a look at the BJCP.
I looked for styles that have American in the name or an allusion to them being an American style in the description, and this is what I came up with:
1A, 1B, 1C, 2C, 4A, 6A, 6B, 6D, 7B, 10A, 10B, 10C, 12B, 13E, 14B, 14C, and 19C.
Of these 17 styles here are the number of times that some of the more mentioned breweries were listed as examples:
Bell's: 7
Sierra Nevada: 6
Rogue: 5
Deschutes: 4
Anchor:5 (Not really mentioned here, but when I noticed it kept showing up I started counting them)
Also worth mentioning is the fact that you can remove the first 9 categories and Bell's and SN each lose only one example.
Obviously this is not really prove anything one way or another, but I thought it was interesting information that I'd share so other people didn't have to spend as much time on it as I did.
EDIT: I was torn about including RIS, since it's not a classic American style, but the BJCP does acknowledge that the American craft brewers have taken over the style recently. However, if you think it should be included Bell's, Deschutes, and Rogue each gain one more.