I realize that this usually means a bad seal but is there any other possible reason?

I only had time to check the rear rotors compression today and it seemed to be 90-100-90 (I want to test it again to be sure). The motors pulling 14 inches of vacuum at idle.

 

Sure, vacuum leak, blown LIM gasket, etc... However, doing a compression test is the easiest thing to start out with. I would test the compression on the front and retest the compression on the rear.