User blog comment:FairyRaven/Fairy Tail Chapter 423: Do It All for Love, Review and Commentary/@comment-87.202.75.211-20150309114538/@comment-7418318-20150315083949

You heard wrong. This was never told or even hinted at in the official material.

And neither is it plausible in my opinion.

We've seen time and again that training makes mages stronger. Just look at everyone not caught in Fairy Sphere after the 7year timeskip or Natsu and Lucy after the 1year timeskip. Surely you don't believe that they all unlocked additional magic containers, right?

So if the ones not treated by Ultear had gained this training boost and unlocked their second container in addition, their power-boost would have been much bigger than that of the ones only treated by Ultear. But that was clearly not the case.