the seven is a symbol for "alkaline" they started to use it after they got their own label. they've had way more than seven cds... a lot more, so that is definitely not it. haha.
Yes, they’ve had more than seven releases, but only 7 official studio albums, not including splits, compilations, re-releases, ep’s and b-sides. Also you can see in this interview, and several others, Matt himself say that they were thinking of calling the record “7” and that Derek came up with the symbol for it.