What Aces n Eights said.
A one ear, split ear or slip ear headstall is designed for use with a curb bit. When you engage the reins on a curb bit, the upper attachment of the bit (the purchase) rotates forward, tightening the bridle over the poll.
A snaffle bit, when you engage the reins, pulls up on the lips in the corner of the horses mouth (among other actions) which makes the cheek pieces looser. A headstall with browband and throatlatch is the proper equipment to use with a snaffle bit, as it keeps the headstall stable and on the horses head.
If you engage the reins on a snaffle bit hung on a one ear type headstall, the bridle will loosen enough to come off the horses head pretty easily.
I’ve seen bridles fall off, sometimes from a hardware malfunction but mostly when someone is using a snaffle bit on a one ear headstall.
Ive heard some unkind comments about people who are unaware that a snaffle bit wasn’t meant to be used with a one ear headstall… but I will just explain why.