Ok. I explain.
Fans work by creating a vacuum when the blades spin, the byproduct is the exhaust out the rear. Vacuum is pretty much uniform across the entire front of the fan. So in pull config, you get even airflow across the entire circle where the fan connects to the rad. Basically a solid flow.
With push, the exhaust of that fan comes out directly from the blades. If you have 30cfm intake, you get 30cfm exhaust. But coming from the blades, it's almost 90° to start with. Unfortunately, there's this big chunk of motor sitting right in the middle and that sees exactly 0cfm. So you end up with a donut shape of flow through the rad. Same airflow, less area across the fins. To combat this, you'll need higher rpm to get the same results as pull. That's why at @1200-1500rpm for most fans, you've reached equilibrium, enough donut flow at speed to equal the solid flow at lower speed.
If you've ever looked at a dirty aio in push, the dust donut is quite obvious, whereas in pull it'll be a more even coating of dust across the entire fan area.