The tadpole is just a convenient plot device. Apparently it makes characters like Wyll, who's supposed to be a renowned hero go all the way down to level 1. It just raises a bunch of questions that Larian probably didn't think about answering.
Yeah it did the same with Gale.
But it is just how RPGs work. We didn't really need an explanation. Like in Dragon Age, Varric is just level 1 again! No reason, don't worry about it.
