i really think some guys 'attachment point' of the penis is lower than some others. Ever bulge watch? Some seem to be at the bottom of the pants zipper, other bulges seem to be midway or higher up the zipper. I'd guess belly buttons vary too. Don't know where I fall on either, but when I'm really roused up it'll stab me in my belly button.
As a young (but over 18) lad I had to wear my shirts untucked as my penis would randomly take a peek out & over the waist band of my briefs & jeans - periscope up.