Ok can confirm first ten chests PER BNET ACCOUNT give 6 shards per day.

After those first ten its junk and rarely 5 shards.

I have opened 18 chests on 4 total alts across 2 WoW accounts but all on the same Bnet.
first 10 chests gave 6 shards each
2 chests out of the next 8 gave 5
6 gave nothing but trash after the initial 10.