The average behaves differently at the saturation point, before and after it.
At saturation, the average plunges because the highest scores in the tier history are taken out, one by one. There might be a 75 to eliminate, the equation would then be (50-75) / 500 = -0.05 on the display.
After saturation, the worst of 500 scores would have to be close to 60 to cause the reaction you describe - improbably close to your average.
Right before saturation, your 50 (=2x25) would translate to (50-60) / 500 = - 0.02 - that's what you describe.
What was the average's reaction on that recent 33? Did it move up? If so, you're not saturated yet. If not, then you are.