yeah cause last season you start with a blank slate with (from what i hear) mmr of silver 1-3 or something
mmr from the last season is taken into account so your placement matches are COMPLETELY ludicrous although you might get players above your skill bracket by quite a large margin (i had a plat 2 who dropped to gold 3 teemo against me in my first)
last season i started b1 after going 4-6 in placements, ended silver 4 with a 48% winrate or something started playing in may and went into ranked in july i believe
this season i started b2 after going 5-5, makin my climb now :)
also from what i hear your earlier games in promos make more a difference that the later games
so if you win the first 5 you will be higher elo than if you win the latter 5 which happened in my case lost the first two the fourth sixth and ninth games
hey dont worry about it too much, you know what they say, if you cant climb then you probably belong there
and with that i bid you good luck and have fun
ps. if anyone sees anything wrong with what i said please tell me i will do my best to correct myself in the future