I think the Federal Champion Ammo uses plated jacket bullets... Maybe they are afraid that the plating will come off in the slots...
jb is so smartI got an email back from Federal today. This is their response:
The Champion loads have a thin copper wash on the bullet that may
possibly come apart in your firearm exiting through the ports and is the
reason for such warning on the packaging. Both boxes you posses contain
this same bullet design.