">>Because the threads on commercial and mil-spec tubes are produced differently, there are slight variations in thread height and measurement. This usually means a commercial tube won't fit a 7075 T6 forged (mil-spec) lower receiver. And a mil-spec tube likely won't fit a commercial (6061 T6 billet) lower, either."

That's what happens when you buy the cheapest parts and end up with stuff that was originally (or should have been) scrapped but someone decided to sell it/use it anyway. Over decades, I've been occasionally horrified by what shows up in varioius name brand guns. When things get tight and cost cutting begins, it usually starts in the QA/QC department.