Kelemelan gave a lot of good answers, but I would like to sum it up. 🙂
In Season 1 each pack had it’s own base model, with it’s own items. You could only use the additional backgrounds, background items, card templates etc. for each pack.
In Season 2 we only have 3 bases – sofar, it may be more in the future – : Female, Male, Muscular.
But the reason for this is, if you buy Female Elf and Female Modern packs for example, you can mix and match the items. Your pointy eared skin can wear a business suit, with a magic sword in her hand.
To be able to do this, all the skins have to match pixel for pixel. We have a bit of leeway at the head, but all else measures the same.
Sade has cool tricks with skin tones and shading to make them more slim or muscular (I like her work on the drow females the best), so you will find some skins that look different.