It depends on the woman's position, if they pull their legs towards their chest it makes the vagina shallower, enough to be able to rub it with your fingers.
With doggy style, gravity seems to pull it out of the way (unless you have a downward curve) so an 8"+ doesn't impact it as much in this position.
Missionary position also helps avoid it because the penis is rubbing against the anal wall whereas the cervix is more on the stomach side of the vagina.
The cervix doesn't create the depth limit of a vagina, rather it is the stretchiness of the vagina skin above/behind it.