Fairly sure devs put a cap of 1.267GHz for the highest freq users can set for the most part. Most that OC that high do get instability issues, freezes, reboots etc, but some EVOs are just stronger than others and can function at this level.
I can OC my Evo to 1267 with no problems. I run 1228 daily and I never get a FC, reboot, or anything. Then again, my buddy Jerzy get's instability issues up in the 12xx's. It's based on the phone but I can OC to 1267 and it runs beautifully.