2 different Penis sizes?
It is playing catch up
Penis erect size varies depending on how aroused you are, most guys will vary as much as a half inch or more.
A lot of things can influence it however the best way i can explain this to ya is think of it like blowing up a balloon you have to work harder to blow up a cold balloon so it takes more effort However if you try and blow up a warm balloon that has been blown up a lot recently its very easy and requires less effort. Your body is limited to how it sends blood down there to inflate your member so if its like the well used and warm balloon its easier for your for body to get blood down there and make it swell. *disclaimer* This is not the real science of what is happening but it does serve as a very simple way to understand whats going on