I've seen mobs drop more then one type of shard or crystal on a regular basis. Some mobs I just can't get the right nuke for. For example with coblyn, right when I researched and saw the type of shard/crystal it tends to drop, I use the corresponding strong element against it and get meh results. I do the same thing with a Puk and my starting nuke can knock off 40% of it's life.

I've visted YG but it's data tends to be wrong. I guess data hasn't been updated over all the patches.