In my experience, not everybody can achieve that super tight, never-break-no-matter-what-you-do type seal.
Picture an ear where the non-canal portion is small but deep - it makes logical sense that it would have less "wiggle room" for the IEM to move back and forth, thus possibly interrupting seal, compared to a large but shallow ear.
Aside from that, I've noticed quite a bit of variation in the ears themselves, regardless of shape or size. Some ears are really solid and hard, others are droopy, flappy things. I assume that would make a difference too.
For mine: when I stick my fingers in my ears and open my jaw wide, I can feel some major changes in the ear canal. It opens up significantly. It would be impossible for acrylic customs to fit in such a way that is comfortable while my mouth is closed, but still fills that cavity when my mouth is open. Silicone customs don't quite do it either, except for my Earproof Atoms which are made from an unusually soft and squishy material. But even those break seal occasionally. The important part is that my seal is always fully restored when I'm done yawning or whatever it is.
Bottom line: don't feel the need to compare your experience to everyone else. If they feel comfortable, and meet your needs most of the time, that might be the best possible solution for your particular ears.
