I don’t know either.
As I understand it, it’s like having a container with a million ping-pong balls - 997,000 red and 3000 green (based on the relative proportions of U-238 and U-235).
Then, you start pulling balls out of the container. If you get a green ball, you put it back; if you get a red ball, you throw it away.
Initially, nearly every ball you pull will be thrown away. Eventually, though, a significant proportion will be green, and you put it back.
Clearly, as the number of red balls decreases, you’ll be disposing of them at a lower rate. Why would uranium atoms be any different?
You assume the balls are the same size. I will bet that is where the difference lies. As the heavier atoms are flung to the outside, it is easier to collect them. Just a thought.