 The formula for the VGA horizontal frequency is thus (60 ÷ 1001) × 525 kHz = 4500 ÷ 143 kHz ≈ 31.4685 kHz, obtained in practice by the method common to all raster-based computer graphics of using a particular crystal oscillator or PLL frequency for the pixel clock and deriving all other horizontal and vertical frequencies by integer division.In this case, a 25.175 MHz or 28.322 MHz pixel clock, counting 800 or 900 pixels per line, arriving at 31.46875 or 31.46889 kHz (and from there, 525 or 449 lines to arrive at 59.94 Hz or 70.09 Hz); the tiny mismatch (7 to 12ppm) is easily accounted for by, and is well within the limits of the retrace synchronisation system.In fact, CGA itself was further adrift (as it could only scan 262 lines per progressive frame, instead of 262.5 to match NTSC's interlaced nature), and the limited precision of quartz oscillators - and their tendency to drift slightly with temperature and supply voltage - means real cards will have slightly higher or lower frequencies whose variance vs the nominal figures can easily exceed the inherent mismatch.