yeah, its certainly possible. The science behind it is that, firstly everyman will have a limit to how big they can be, but they may not always reach this peak in their size. The main reason for this is the amount of sexual arousal, which the higher the mans sexual arousal, the more blood goes to his penis and so he will grow harder or larger because of this.
In my case, i see there being 4 stages of hardness for the majority of men,which are as follows:
1. Flacid, when their is no arousal i.e. how most men will be some of their day.
2. Semi-erect- this is normally when their is some sexual arousal, but not a huge amount, such a level of arousal could be simply because of a penis rubbing against the lining of underwear or trousers etc.
3.Erect- This is when a man is sexually aroused and this is the size the man will be in the majority of sexual encounters.
4. Peak Erect- This is when the mans penis grow to the largest it can. This will normally occur when a main is extremely aroused, and for some may occur in a lot of their encounters but for most men, this will happen with in a minority of their sexual encounters.
At the end of the day, the difference in size is based upon the amount of blood that is sent to the penis, which normally varies depending on the amount of sexual arousal that the man is experiencing.