It seems logically you would have some sort of comedown everytime, because from MDMA your serotonin gets depleted right?
Every roll happens under different conditions, i.e. different dose, different setting, different levels of exercise, sometimes even different chemicals. Even if all outside conditions were equal, there's no guarantee that your roll would be the same every time because your brain chemistry changes from day to day.
from my understanding even over a dose such as 120mg can completely deplete your serotonin
That's not true, for most people a 120 mg dose of MDMA will use up a large amount of serotonin, but not all of it. Without any serotonin, you'd be dead.
If by "completely deplete," you meant "deplete enough to guarantee an unpleasant comedown," that's also not true.
i've never experienced a bad comedown yet, only afterglows, but then again the whole afterglow doesn't make sence to me either due to serotonin depletion.
If you've been using MDMA for a while, this means you're probably doing it responsibly, which is good.
Most people who experience unpleasant comedowns find it happens after using MDMA repeatedly over time (gradually depleting reserved serotonin and, as JWills noted, influencing receptor density), not in the first few uses.