TV picture is refreshed on screen with frequency 25 frames per second or (when double frame rate is turned on) 50 frames per second. Display picture is refreshed with frequency set on display properties from the list of rates supported by you display. When frame rate of display is not divisible by frame rate of TV, it will appear the beats that looks as jerking of crawling line. In theory the beats will be on every display refresh rate, but on 75Hz the beat frequency is sufficiently high that subjectively reduces the visual perception of beats as jerking.
Also when double frame rate is turned off, the 25 Hz of TV frame rate become visible. You may not see it on TV set because the TV picture actually displayed with 50 fields per second.