Meyer only wanted to sign 28, but this guy is the kind of talent that you can't pass up.
The NCAA caps the number of scholarships a team can have. Currently, that number is 85. As I understand it, Urban Meyer wanted to give out 28 scholarships because that is the max number available for this class. If this is true, then there is no room to take in Seantrel without taking away someone else's scholarship. In general, coaches do not like adjusting their magic number due to the hassle and potential for NCAA scrutiny.
If 28 is the magic number, but the Gators left an opening or two in case of emergency, then by all means, do not pass up on the guy.