I have 80% in skill now.

Means that "normal" doesn't come into it.

There may be a low probability of 7 successess without improvement, but it's still possible.

I think my last assassin went something like that at one stage.

Fact is, if you can land the assassinations, don't worry so much about the skill. That's the irony of assassinate; those who can land the assassinations need the improvement less than those who can't, but it is the latter who will not see the improvement. The former will see it but were dangerous without it.

But for me it started at 82%, IIRC.

Then it took 13 or 14 more to get to 84%.

This was in late '09, and at the time I was very confused as well, because most of the logs around I saw improvements every couple at most.

They changed it so if you assassinate the same person you won't get improves from it.

Also if you get to 30 assassinates with no improves while reporting it as a bug and they say there's no bug they probably just really don't like you.