I found track number changed

Hi everybody,
I have a problem that is driving me nuts, I tried to look out for some solution online but nothing worked.
Basically I am formatting my mp3 collection and I am doing the conversion of the tracks number from 1 to 01, etc. And everything it is ok but if after a while i go re-edit the folders I already done I found that some track numbers, randomly, are changed in something like /1.

I have windows 7, I use iTunes and songbird to listen the music, but I cannot figure out who is that change the mp3 tag.