Why is this cut done to heads? one of mine has it while the other does not.

i looks like its been cut to suit a 94mm bore kit.
rick
manx020

Unless you mean the top surface, which is cut in order to give the flycutting mahine a flat surface to work from. Maybe one was flat enough, or they forgot. American heads are usually machined by poorly-paid mexicans. The machinery is precise, but the operator is probably worrying how he can pay back the loan he took out to take his son to a doctor.