Had free GarageBand now it's the full version. Never paid for it, what happened? I had free GarageBand on my old phone, didn't use it for a bit, went back to it and now it's the full version...

I never paid for anything, and I don't believe in a "free lunch"...

So what's up with that?  I honestly have no idea how this happened?

Should I delete it and get it again legitimately?
If you have gotten a new 64gb iPhone starting with the 5s, iWork and iLife apps come pre-installed. You don't have to purchase them once you purchase a new device. The same is true for an iPad.