Ok so your mmr from last season is taken into account when doing placements in a new season. Those who've never played ranked before get a starting mmr of 1200ish (silver 4 or so), but people who played ranked previously have a soft reset pulling them towards that mmr, but are still pretty heavily influenced by their mmr at the end of preseason. As a result fresh ranked players can go 6-4 and end up low silver, while challenger gods might also go 6-4 and end up in high plat, and bronze people might end up in low bronze, despite also going 6-4.
By the end of season many people end up with mmr's somewhat lower than their actual final division (due to how forgiving demotions are), so it's quite common for people to do their placements and do well in them, and still get placed quite a bit lower than they were expecting based on their final rank last season.
Going 6-4 in placements and getting bronze 5 makes me think you were probably mid-low bronze last season, with an mmr reflective of that.