User blog comment:Youske/Episode 140/@comment-114.74.201.9-20120721043712/@comment-5782519-20120721053258

Hmm, well the prisons inside stop you from using magic, not raising your own magic. Like how Lucy was doing inner-control training? Not too sure... XD We don't know how long they've escaped for, so maybe they were gone for long enough to learn new magic.

I find it unlikely they were in their cells for seven years. The priest that visited them was clearly planning something evil, so it'd be bad to not let them get stronger as Mirajane has shown in the past, inactivity for a long period isn't a good thing (although Mirajane was still super strong anyway when she could use her magic again). :D