Mid-silver matchmaking

I know, people gripe about matchmaking all the time, but I'm less complaining and more confused.

I'm basically a silver 3 support main (currently silver 4, 88 LP). For this entire season—about 100 games, most in silver—I've noticed that when I'm in S3, my teams are substantially worse (compared to the opponent, obviously) than when I get demoted to S4. I've been demoted... three times now? Maybe four. Every time I drop, I'm back in promos with at most one loss on my record, and except for this past one, I win a promo game and make it back to S3.

By "substantially worse", I mean in S3 I routinely have those games where your mid Xerath dies seven times in fourteen minutes or your lane partner thinks you can all in when the opponent is under turret at half health. The situations where at least two teammates feed so early that, even as a support with more carry potential (Zyra, in my case), the tide of the game has turned too far by the time you're capable of impacting the rest of the map. But in S4, the vast majority of my teams handle their business and at most we just have to gain some small advantage in bot to carry our weight. I neither claim to be great in the S4 games nor awful in the S3 ones; I'm basically playing the same game.

Is there some kind of an explanation for this? Does it happen at other divisional cutoffs? At first I thought it was because of early season placements frequently being placed in mid-silver—which may have been partly the case, given how many smurfs I saw—but it's still happening and the smurf problem is mainly gone.