grindi, remember that classification of players has more to do with overall rating than how good a player really is. Typically, there's a strong correlation between overall ratings and GUESS ratings, but not always.
Also, top ranked AA teams can see LOTS of BCS-caliber players. They rarely get to sign any of them because they can't compete against BCS teams, but good players do occasionally slip through the cracks and go to AA teams. As an example, last season with my elite team, Florida State, I found the #17 RB in the country according to the GUESS rankings considering only an AA team, and this was AFTER signings. He even had high potential. Needless to say, I stole him from that AA coach who'd failed to sign him for some reason. How that coach ever even had a chance to sign him is beyond me, but somehow he did.
DI-A teams will also grab players considering AA teams as backups. They don't always sign those guys, but they're there as a safety blanket. If they don't end up signing them, they'll go back to the AA team that had them green previously.
Also, I often leave 1 or 2 scholarships open until after signings and try to find a cheap guy who's gone back to "undecided" after signings begin. Occasionally, I end up having to snag an AA guy instead. He may not be a stud, but he's cheap and he's better than a SIM recruit, and your guys can't all be 4-star players afterall.
Oh, and it's MUUUUUUCH cheaper to take a guy considering the AA national champion than it is to take a guy considering a crappy A team.