Hi, i have exactly the same problem but my hardware is different:
Asrock Z77 extreme4/i5 3570k/gskill ares 2133 9-11-10-28
exactly the same issue as you describe, i tested my memory modules on A1 B1 A2 B2, and i have the same timings issue on B1 and B2, the timings indicated with memtest are...