When you complete a matchmade game your stats are recorded for the matchmaking playlist that you played in only. These stats are then used to calculate your experience for that playlist. The end result is that each player has separate stats for each ranked matchmaking playlist. These stats are summarized in the form of a "level" for each playlist. If you've never played a game in that playlist you will have no level. New players start at level 1, the highest level is 50. Most players have levels closer to 10.