24 is the average. MMR will impact this. If you’re below diamond, then PBSR will impact it as well.
No, they don’t. What you’re observing is your MMR.
Looking at your profile at this very moment, your tank SR is 3175. Let’s say that is exactly what your real rank is, as in you deserve to be there and precisely there.
If you win or lose a game from that point, then the delta will be less than 24 (so let’s say you’d lose 20, or win 20). The reason being that the system wants to try to keep you at 3175, because it has calculated that you perform well about there.
Let’s assume you won. Now you’re at 3195. Your SR has changed, but your MMR hasn’t. The matchmaker currently believes, based off of a large number of factors like your previous ranks, average performance, how often you play, and so on, that you should still be at exactly 3175, but you aren’t. You’re at 3195.
Now, let’s say you win again. But, tank queues were kinda janky because you were playing off-hours, so you were the only diamond and everyone else in the game was high plat. The game doesn’t perceive this as an “earned” win. It expected you to win because you were playing down into a lower rank, so it doesn’t change your MMR. It still believes you deserve 3175.
But your rank still does increase, though this time though it’s by even less, say 17 or 18.
This is a bit exaggerated, MMR doesn’t actually use SR numbers or ranks, and the changes in SR delta wouldn’t be that abrupt and gravitate around an exact point, it’s most likely a large range of values with other stats thrown in. For instance the game might believe that your actual rank is within the range of 3100-3250, and you’re expected to rise and fall within there fairly normally.