AR-Stoner 6.5 Grendel barrel short chambered.

[dmsims21]

Just like he said, it was a tight throat and Bill had to ream it out. He should have made these measurment changes then gave it to SAAMI.
Steve said he knows the issue and might be making a modified Grendel Reamer with the wider throat. So it's not wrong, it's the way Bill A submitted it to SAAMI.

That doesn't prove the design is wrong. It may, or it may not. Another explanation is that the throat was not in spec or it was a little rough.
The chamber design is probably difficult to make reamers for, and difficult to inspect. You can't just put some calipers on the throat area and measure. You have to measure at a specific location along the axis of the reamer.