I think the reason bits are supposed to give one wrinkle is because of the bit's positioning. If the bit is too low (i.e. no wrinkles) the horse can easily get the tougue over its bit and also, the bit can bang on the horse's canine teeth (if a gelding). Plus, the bit will move around a lot more as it won't be as secure. That's the reason for the wrinkle, loosie. Its odd that nobody you asked knew the reason.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare