[Gameplay] the spell vamp ratio on mordekaiser q is incorrect

ive recently started playing mordekaiser and i hear he has many bugs. I noticed that, when he has hextech gunblade hes not healing for the full 15 % on his q's. i thought it had something to do with the health cost, but no the health cost is only on cast. he seems to heal for 8-9% instead of 15. it seems like a simple issue to fix. or maybe it something that shouldnt be fixed since he gets "spell vamp" from his passive, and this could make him alot stronger