It's because you are still in B3. I know how random the games are at this level and you might get stuck there for a while, but basically B5 is full of people who don't even care and B4 from people who struggle with basics. So you can play pretty bad in B3 and have you mmr fall pretty low without feeling any menace to fall into B4 seriously.
If you are a bit unlucky with your matches (akf,feeders,smurf...) the transition from B3-B1 is really noticeable in term of skill. If you ask me, the only distinction between B1 and S1 is the amount of lucky noobs who managed their way there without the proper skills. Don't give up, your win streak is a good sign that you are getting there.
Don't forget that the most important is between your ears, not in a number on a server.