From the looks of it that account did not rank last season and as such I would assume that the system is adding last seasons mmr 0 +1200 /2 so giving you 600 mmr. As it had nothing to go on form last season it's guesstimating your mmr and putting you in B5 as it had nothing other than your promos to go on.
However if you keep playing like you are now you will get up in the rankings in no time. Just keep doing what you are doing bronze is not the end of the world it's just part of the learning curve / grind :) I expect to be tossed back into the Bronze division and have to start teh slow grind but I picked up the game just last year so I hit my goal of getting to silver... this season goal is gold so a whole season to go up to gold seems viable being the 2nd season I play!
-set realistic goals
-set specific parts of play you want to improve
-try to play a small champ pool
-try to stay onto of changes and try to understand why pros do certain builds / plays and try to use them when viable.
those are the 4 main things I'm going to try and follow this season.