In opening many many legendary hero cards, I have only gotten shard heroes. I have opened 5 event cards. 3 cupids and 2 snowzillas. If you think about it, how many paladins have gotten from lhc, and how many skull knights?
Open much more LHC and we can talk if there is pattern or not. True is that it is really hard to find SK, but i have found every other hero from LHC. It's all about RND system. One can get only shardable heroes, other will receive everything else.
The notion that random means equal probability of all outcomes is quite misguided.