That is the downside of a CNC program. Sometimes the casting variance will move features around but the program is the program. The more aggressive the CNC program the more likely you run into thin spots. I had Modern do a set of heads for me years ago and their program was so aggressive that the heads ended up needing to be repaired. On the flip side, the Trick Flow programs seem to be conservative enough that I've never seen any problems with any of their heads which have come in the shop. It just boils down to how aggressive the programmer was in removing material to make power. And of course, sometimes it is just bad luck. Even a conservative program can end badly if the casting is thin.