Jeanne is correct. I don't think you have a horned bull that had a good dehorning Job. Sounds like you have a hetero polled bull with some hetero polled cows.
If he was truly a horned bull that was dehorned, 100% of your horned cows would produce horned offspring when bred to a horned bull. If you have even 1 horned cow that had a polled offspring, that means he is a hetero polled bull, not a dehorned one.
If he is hetero polled bred to horned cows 50% of the offspring would be polled / 50% horned.
If he is hetero polled bred to hetero polled cows 75% would be polled (25% homo polled/50% hetero polled), and only 25% would be horned.
If he is hetero polled bred to homo polled cows 100% of the offspring will be polled with 50% homo polled 50% hetero polled.
If you have a homo polled bull, all cows will have polled offspring regardless of whether they are horned, homo polled or hetero polled.
When home raising bulls, or raising composites, it is important to get a blood test to confirm they are homo polled if you don't want horns.
We could go down the rabbit hole of the African horn gene, but it doesn't sound like that's an issue in your herd.