Osu automatically updates player rankings once per day, but every time you make a new play it manually updates your score. This means that sometimes when you first log in the ranking that you see is inaccurate, and usually higher than its supposed to be. When you submit a score osu fixes it, so you may see -100 and -200.
Also, accuracy plays a bigger part in the pp calculations that you may think. Even if you have a higher combo, you may still lose pp, if your accuracy is lower. For example, if you get 98% and 480 combo on a map, you may get more pp than if you score 94% 500 combo fc. However, osu will keep the second, lower pp, score because the score was higher. Combo gives you score, but it's ultimately the accuracy that determines the amount of pp you get.