Cause Ziemba is going to have to play guard in the NFL, and may have a hard time doing that:
http://www.cbssports.com/nfl/draft/players/1264629
The Bills had already drafted an RT in the 4th round. Also, I think things change a bit once the later rounds come, short term expectations are lowered for the pick. The Bills had a need for a 5th CB, I find it hard to believe you would draft an OT in the 7th round and expect him to challenge as a starter.