Immolate/Wither ticks count

I’ve tested Immolate and Wither ticks and noticed that for 1 cast Immolate makes 7 ticks but Wither makes 11 ticks. I’ve tested it multiple times with same main-trees talents (without procs like Flashpoint and Reverse Entropy), without gear (with gear numbers are also different - 10 and 14).

Why there is such difference? Both spells have 21s duration. Hellcaller spec have no +haste hero-talents if I remember. Is it a bug or smthng? Ticks count is very important for shard generation at least.

Upd: 1 full-time cast of Wither always gives more than 1 full shard. 1 full-time cast of Immolate always gives less than 1 full shard.

In that way Diabolist spec looks even more unplayable than before, lol