XYCat: I just don't get the volume 1 and volume 20 thing
As HyperinsomniacLive noted, it's intended to get you interested enough in the series to buy the others. This doesn't fully explain issue #20, but they state on their site it's a chance for people that are already following the series to get the latest issue digitally if they don't already have it.
It's not all the uncommon to do things like this. I've seen it for DVD series where the first season is discounted, and sometimes the currently released season will be on promo as well for the fist day or days of release.
Grargar: - The Manhattan Projects Vol. 1
I quite like this series. Enough that I've picked them up in hard copy.