US Army came up with Enhanced Performance Magazine. The new M855A1 ammunition were impacting feedramp when feeding, so the new magazine changed the magazine lip geometry for better feeding.
The new magazine will be phased in as the older ones are cycled out.
On AR15.com Magpul posted that they are
seeking to get the test data which supposedly show better poerfromance by M3 PMAG
From:
The Firearm Blog,
Soldier Systems Daily, AR15.com