AFAIK It's determined by the Agnus/Alice custom chip version.
First agnus was ntsc only, the next two were available in pal/ntsc versions. From ECS onwards the chips are the same, the motherboard puts either 5v or ground on a pin that sets the default & you can change it in software using BEAMCON0.
It's actually misnamed as PAL/NTSC covers the colour encoding. It just changes between 50/60hz.
PAL60 is the common name for a 60hz signal using PAL colour encoding, most modern TV's support it. Some don't which is annoying if you have an xbox360 as not all games will run in 50hz.
