So I qualified in team arena for Gold four. I, if I remember correctly, won 7 out of 10 games (I lagged out in one of them which counted as a loss). And for team slayer, I qualified for platinum 4 even though I did much worse than I did in arena (I lagged out of I think 2 games and loss more). So, I got ranked an entire tier higher in the playlist I did considerably worse in; and this makes sense. Only reason I can think of as to why I was ranked higher in Slayer is if, #1, H5 takes into account your KDR, which was maybe lower in Arena since, when playing an objective, I don’t care if I die defending strongholds/flags. Or, #2, H5 takes into account the number of assists you got.
Your MMR doesn’t carry over between playlist. You have an MMR for Team Arena and one for Team Slayer.
That said, you’ll want to check out the MM feedback archive pinned above.