No real Rymth or reason But generally
X.y
X = Major updates over a lower X (new features)
y = Minor updates over a Lower Y (old features fixes)
Sometimes ill update X if its a scipt that is only updated very infrequent .. like my GM detector .. updated about once a year so what ever update minor or major it gets a +x
If thiers been a significatant amount of Y increases ill bump to a new X, even if no major new features are added.
Depending on the shift of the moon, the color of the sky, the ambient outside air temperature, what side of the bed i woke up on, if i had my tea yet, how many phone calls i have gotten or other misc infleuences all of the above is chucked out the window and pure random x,y increases decreases can happen.