LP gain getting lower after WINS?

Short story time:

I got promo'd to Bronze 3 the other day. My first win got me 23 LP. My second win got me 23 LP. (I'm on a win streak here) My third win got me 20 LP. WHAT?! Then I've been getting 20 LP since, and I'm losing 17 LP per loss. Regardless, I'm managing to win enough to reach promos, but it's pretty disheartening. Anyone have some insight into how my MMR is going DOWN even though I'm winning more than losing and my grades are B+ through S- consistently?

For those that are bored, here's my theory:

Season just started, so everyone got dropped a rank or a few ranks depending on placement luck (I had 2 games with leavers and 2 with inters). My theory is that my MMR may not have gone DOWN, it's just that others are playing more and climbing quicker, raising the average MMR at each division. So, for example, let's say when I reached bronze 3, the typical MMR of a player here was 700, and mine was 750 (Making up numbers for sake of illustration). I won a couple games to give myself some cushion and got off for the night. When I then played again a couple nights later, perhaps the Season 7 Bronze 1 or Silver 5 players that got placed B4 in S8 due to bad luck have now climbed out and are in B3. If their MMR is higher, let's say 800 - 850 then I'm now below average MMR for being in B3. Of course, nobody knows for sure how the MMR and LP systems works and every single factor involved but this is the only explanation I can think of. Perhaps the Riot Gods will bless me with an official response on how someone can see reduced LP gain mid-win-streak.

Thanks for listening!

-FuzzyWumpit