I have never heard of Federal doing anything other than thier match ammo.Fact is, it has been few years and I have gotten out of the loop.At that time,that match ammo was about $11/box.
Figure if the rim thickness varies, the round goes in deeper or not so deep into the chamber. The firing pin strike is then not consistent.
Since the rim is the headspace, the firing pin will drive the thinner rim in deeper.Just as if you size a center fire cartridge too short and the shoulder has to go forward into the chamber until it contacts.
Unles the Winchester stuff is the same lot number as what you tested,it could vary and not help much. Be sure to check that.
