Like us females, all bitches are different. Neneh had her first season at aged 8 months, and goes approx 7 months between, whereas Meg was 14 months old when she had her first season and went 9 months between.
I'm sure the vets line of thinking is that it is suggested the optimum time to spay is midway between seasons, ie: if a bitch has a 6 month cycle then spay 3 months after the season.
This is all to do with the hormone levels as the oestrogen & progesterone rise & fall like peaks and troughs, with the level being its highest when the bitch is in season and at its lowest midway between seasons, after which it then starts to rise again.
It has also been suggested that to spay midway can sometimes help preserve the coat a little, but I can't personally vouch for that.