Here stand how NTSC work.
My values are 0.3% diffrent. I have H=15674.1749 Hz but spec for NTSC say 15.734 kHz.
http://en.wikipedia.org/wiki/NTSC
NTSC (National Television System Committee) specs are:
Transmission Band UHF/VHF
Lines/Field 525/60
Horizontal Frequency 15.734 kHz
Vertical Frequency 60 Hz
Color Subcarrier Frequency 3.579545 MHz
Video Bandwidth 4.2 MHz
Sound Carrier Frequency 4.5 MHz
on PAL there is a odd and a even frame 1 frame is 312 lines next frame is 313 lines. so line length toggle each frame by 1 to reach 625 lines .On ntsc its same i guess because 525 is not dividiable thru 2.