Got this from the uesp:

"Setav - If used on a skill, it will increase the skill without causing inadvertent level-up issues (like with "advskill"); the expertise level message does not show but the expertise level is increased or decreased correctly."

Does t his mean that the difference from current skill at say 80 if I setav to 100 will NOT consider the 20 increase for level up?

Having not used the console in Oblivion at all and having read what you quoted I would say no, it will not count towards a level-up.

On 360, if you use a trainer for, well, training, there is a delay by as much as one skill level (not player level) that that skill will not advance up visually as normal. But I'm told it advances behind the scenes, and when you reach your next level in that skill, I can visually see it advancing fast, as if to make up for that one time of seeming to stand still. Related?