I think one big "variable" is that the woman has learned to relax to the point where she can let it happen. Some I think just naturally "let go" during sex and so it's easier, but others mis-read some of the feeling as an urge to pee so work against it (can't say I blame them). Yes, I'm male so don't know this from my own body, but I've coached women through this and so have found some intuition. Like most sexual things, we are all wired a little different so I've seen a big difference in how responsive g-spots can be. Mostly have noticed that the squirters g-spots swell quite a bit before squirting occurs, and other non-squirters might enjoy the same stimulation but don't swell much at all. Who knows ... it's likely a combination of physical structure and neurological wiring and psychology ... which is what makes us the fascinating beings we are.