On CRTCs 3/4, HSYNC occurs 1µs later than on CRTCs 0/1/2.
==== Discolouration effect ====
On CRT monitors from other brands, a colour calibration can happen just after the C-HSYNC.
So even when a full valid C-HSYNC signal of 4µs is emitted, an HSYNC shorter than usual combined with a coloured border can induce a discolouration effect on those monitors.
=== VSYNC ===