Hello

Void shards are special because you can summon void champions from them.

Void champions aren't subject to the normal elemental effects whether positive or negative.

The thing to note here is that voids don't receive the negative affects, such as weak hits, reduced damage and crit rates which overall gives them the upper hand.

Im guessing the differences regarding the percentages reflect this upper hand. 

Hope that helps

Regards

1. It's just game mechanic, as said above, the units don't have elemental disadvantige, so no weak hits 

(also some of them are just better compare to regular elements)

2. They are supposed to be rare, exclusive, so when you actually get void epic or legendary (or even better rare), it should be special moment

3. Is conected to 2., just another way how to add more possibilities to milk cash with void packs