Heh, I have the same problem or whatever. You know, after you cum, the head gets super-sensitive. Well, if you keep stimulating the head directly, it seems to completely relax the muscles that hold the urine in your bladder, allowing anything in there to get out. I'm glad I found out about this as a kid in the shower rather than with a girl, lol.
Oh, and ***, you'd have to experience it to know why he couldn't tell. It mystified me for quite a while as a teenager. I just knew it kept my dick from oozing cum for 5 minutes after I got off, so I'd finish my shower-whacking with it.
I had always assumed it was the same for everyone...