I'm trying to tell you that your girlfriend isn't the only one. And that there are more out there than you think. Of course it has a limit, but it's far greater than what you assume, it can double in length (and in some cases can be almost 3 times as long) given enough stimulation and arousal. Is it our fault you fail in that department?
I know she isn't. DOUBLING IN LENGTH!? FUUUU
Unaroused high average depth is 4 inches
DOUBLING MEANS EIGHT
WHAT I SAID.
And this may come as a supreme shock to you, but I'm not basing what I know on my own experiences, I'm basing it on lots of things I have read from different places.