Probably the reason why he's still good at the CB position is because of his OVR, SPD, CTH, and JMP. Things like that are CB-like qualities so the CPU sees that he might be good even though his coverage ratings suck.
For example, I moved one of my WR's to TE and he was like a 90 OVR TE. The reason being was because of his speed, his route running ability, and catching ability. But his blocking ratings were somewhere in the 50's.