Promethor is partially right, but still wrong. When in-game framerate goes below the target framerate during recording it means that computer simply can NOT record and render at that much framerate, as result it goes below that value. But the MOMENT you see lower value fps during recording than your set target is, it means video is generating dropped frames.
Dropped frame is when capture software failed to capture the frame in time so it skipped it and proceeded to the next one, as result making previous frame to repeat itself, making it look for you that video "paused" for a moment or as we call it "lagged". More dropped frames = more lag.
What makes you think when you are playing back the video it's stable 60 fps? Did you analyse the number of dropped frames in it?