It's not the horizontal line frequency, it's about the pixel clock.
TFT monitors are digital displays - they've got to resample the analogue VGA/RGB input. While resampling you'll only get a 100% sharp display when each sampled pixel matches exactly one output pixel: the sample frequency and the phase have to match - the better they do, the less sharpness you lose.
Clock Adjustment is simply the sample frequency/pixel clock; it it's too low you lose horizontal pixels and the image looks squashed. If it's too high you get doubly sampled/bogus pixels and the image gets stretched.
