Why doesn't Bandicam record at 60FPS?

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Hello. I have a problem that I haven't been able to solve for a long time. With almost no dependence on the computers on which I use Bandicam, with recording settings of 60fps, it records at 45, 30, or sometimes it reaches 55, for example. Both computers have graphics cards capable of over 60 fps with accuracy.
In the bdcam setting there is such an item as "key frame interval". The default is 150, but the recording is usually closer to 30 fps. If you change the value of key frames, then the FPS also changes, and the number of frames does not increase depending on how large the number of the interval is, that is, if I set 1 - 45 fps, I set 30 - 52fps, I set 50 - 21 fps, that is, fps from side to side changes.
I record from the desktop to mp4. I don’t write in avi - because large file size, although avi (in the file properties) writes supposedly 60fps exactly. Why is that? If anything, fps, which in the end after recording, I check in the properties of the video file.