There are two simple reasons that the F-15A is moving down in Air Sim and why the Su-27s will remain at 13.3. And it really has to do with overall game balance and natural tendencies of experienced sim players to gravitate towards whatever plane is the most over-powered for the given day and what side of a match is more saturated with experienced players.
Due to the state of the game 2 years ago with the addition of ARH missiles and subsequent struggle that “red” side players have…the idea that red side players are more skilled than blue has cemented itself pretty well into the game. At the same time, the addition of more competitive or meta defining planes that play on red side has caused former blue players to migrate over as well.
12.0 - 13.0 is especially bad for this with the addition of the F2A ADTW and the fact that essentially nobody that is even halfway decent has played or done well in the F-15A in recent months. Taking F2A and German F/A-18 against teams of inexperienced US players is one of the easiest ways to farm kills. In fact a lot of former top tier players have migrated to playing 13.0 tier.
I will use my friend SimplySinful as a baseline. The premium F2A is going to be his most played plane at this 13.0 tier and pretty much all of those games are against blue side players. If you look at any of the guys that I commonly play with then you will see that the majority of the time at this tier we are on red side.
This is the scoreboard from his last stream. Note the team composition is mainly F2A and German F-18.

Let’s check another fairly well known and experienced top tier player…Alex137_rzt and see what he plays at this tier.
German F/A-18C
His squad mate DERREISE115 that you will see with him most of the time.
100 games in the F2A vs 8 in the F-15A.
Now let’s look at some older players. How about Starsss2_TV? He is an older sim player, one of the highest ranked TSS players in sim, and has been around for a long time. He is the guy that I showcased F2A matchups in one of my videos.
Once again a very similar story.
Let’s look at another former highly ranked WT TSS player and DCS player…his name is perts.
And would you look at that…the F2A is at the top of the list.
Where’s @kizvy is a question we commonly ask ourselves.
If it’s a 13.0 day…he’s gonna be in the F2A ADTW.
Back when I started playing the game, the REEPRs fancied themselves as the best top tier sim squadron. They renamed to NSFS. Guess what the guy who I would argue is their best player has been playing?
SpitFyreLegend_1
The F2A is the most played 13.0 in Air Sim.

The F-15A is the 4th most played plane in the BR.

It should be noted that the kill statistics for the planes are relatively similar but the rewards for the F2A are far higher on average. The F-15A actually rates the worst in monthly average rewards of any of the 13.0 aircraft.
Average rewards are most likely the primary factor in deciding whether or not to move a plane in the game. If there is a kill / death type component to it, there probably is also more in depth tracking of the highs and lows of kill counts and isn’t just based on a rote average.