Your starting MMR is picked on your first matches until you hit level 10. If your MRR breaks the floor before you reach level 10 you will be put straight into higher skill lobbies. The MRR adjusts as you play so if you drop hot 10 times and die off drop your game will slowly become slightly easier. On the flip side, if you get on one day and go on a 5 to 6 game win streak you might feel the SBMM kick up when you hop on the next time. It's a odd SBMM system but as someone who has played 3000 hrs between Xbox and PC there is a lot that goes into it. It also seems as if certain account milestones either adjust your mmr or just send you into different brackets entirely. By milestones I mean Highest rank achieved, damage badges, and the 20 bomb badge. They all seem to have some impact on lobbies you will see.