That is the KEY rate (FPS/iframes) that means iframes is double the FPS for the camera.
Confirm Mainstream is actually set to both be the same. Sometimes if you change a parameter, it will double the iframes.
If they are then you either have a camera like a reolink and it does whatever it wants or if it is a better camera you are using something like AI codec or self-adaptive or some other variant that is changing the iframes.