Morgana,
The skill hit variation and also monster resistance is taken into account by the game BEFORE the hit number is displayed. So what you brought up would effect the skill hit number. So then lifesteal doesn't match up woth that number. So none of those reasons you gave could account for the discrepancy. It must be a bug.
Re: Lifesteal glitch?
#51A Mage is like a Lamborghini, You don't get one as your first car or daily driver and there are cheaper cars that go faster and are more comfortable anyway.