probably because individual performance matters even less in aram unless you're some crazy teamfight carry champion. it's constant 5v5 gameplay. i doubt the game's systems for determining A and S ranks are as accurate to what creates actual wins in ARAM as they are in rift. a large part of what you're doing to win in ARAM is choosing the right champion to best complete your team in each individual game. if you go for highlight carry champs as often as you can get em, you're more likely to have nice scorelines and good game grades, but probably less likely to actually win, because there will be more scenarios where the enemy team gets a better comp than you because you chose to go with ezreal on a team of 3 mages and lucian when nautilus was available, and the enemy team has a tank, a bruiser, an enchanter support, and 2 high damage carries.
maybe that isn't the case for you and you do pick to round out your team as best as possible, in which case it is totally possible for you to get an unlucky streak of teams in the span of 100 games with 40 wins. it's just more worth spending the time reexamining the way you choose champions and how well you play for ending the game vs getting kills and stuff, and how often that might have lost it for you.
whatever your actual case may be, hopefully you'll have better luck in your future games.