Experience is what moves you to the next level, you pick up experience points with every mission, and when you pick up enough you move to the next level, it is indicated by the purple star. As for why some people with higher levels have less skill points, most people who have been around longer use the workouts to get their skill points up to the highest possible level before moving up, whereas some just focus on gaining the experience as quickly as possible to level up more quickly and end up weaker than others in lower rankings. When I first started I used to try to level up quickly, but now I try to look after my skill points too.