My facial piercings never close

It doesn't matter if I've had the piercing for 6 months, or 6 years, it will never close.
Yeah they tighten up on both ends, but my labret has been out for over 4 years and can still pop a stud right in.

Some of my peers of have had to remove their jewelry for job interviews, hospital trips, etc. and end up having the hole completely healing before they get home.

I like the fact that I can go bare if necessary and not worry about losing my lovely piercings, but I'm very curious as to why? Is there a deeper, more technical explanation than just "everybody's anatomy is different", so I can find and read more?

(All of my piercings were done with 14g hollow point needles; only 1 stretched and it's not included as part of the question.)