10 Wins in Promos = Bronze V for me <3

Because your MMR last year was of Bronze, and your last season effects the current. Still, I'd think with 10 wins you'd get at least Silver. That's just crazy, Riot...

Riot's not been known for their placement skills... Last season I was fortunate enough to get placed in the same division I ended the previous season in.

It's because of your Summoner Name *Kappa

Ok, jokes aside, it's just because when the season ends, it is only a soft reset, so it takes into account your previous performance last season and using the information from your provisionals as well, it places you into your skill range. Since you were placed in Bronze 5 after winning all your provisional games, it means that your skill level is within Bronze 5 range. Sorry, hope you climb outta that Elo Hell (though every division and tier in Ranked is Elo Hell...)

I only won three games out of ten and got Bronze 3. It was also the first time I got into a rank. Maybe I'm decent.

Because the number of placements you win has a very marginal influence on your placement. What really influences it is MMR - a super secret ranking system that cannot be really checked (sites like op.gg show the estimated MMR, not the exact, real one). Your MMR was very low so you got placed in B5.

It's your summoner name.