Why AFMF 2 is smoother than in-game framegen?

So I've had my 9070 XT paired with Ryzen 7700 for a year now, and very happy with the overall performance. Latest drivers and all, Windows 11 fresh install, 144Hz monitor.

One quirk I noticed about framegen:

1. Game runs smooth without framegen.

2. Game runs even smoother with driver-enabled AFMF framegen as framerate doubles.

3. Game doesn't feel as nice with in-game framegen on, almost like a judder/microstutter every 1 or 2 seconds. Framerate doesn't exactly doubles, more like 1.5-1.7x (eg: 60fps -> 90fps with ingame framegen)

I have observed this behavior obviously with WH40K Darktide, Space Marine 2 and SW Outlaws. Has anyone observed the same?