It does sound odd that Lapua would somehow make bad .260 Rem brass. I read a post about Lapua .260 Rem brass not working as well in SAAMI chambers.
A post from November of 2012 on another forum ...
"That is that the case head dimension for the SAAMI is .4738 and the case head dimension for the "Match" is .4723. This is a thou and a half larger for the SAAMI. The new Lapua 260 brass is .4691 at the case head. That is .0047 for expansion. That is an awful lot. So Lapua 260 Rem brass is made for match chambers not SAAMI."
A post from November of 2012 on another forum ...
"That is that the case head dimension for the SAAMI is .4738 and the case head dimension for the "Match" is .4723. This is a thou and a half larger for the SAAMI. The new Lapua 260 brass is .4691 at the case head. That is .0047 for expansion. That is an awful lot. So Lapua 260 Rem brass is made for match chambers not SAAMI."