Well, certainly when you are boasting.
I think it has to do with what you are confident about and how you present your confidence. When someone presents themselves as confident and able to do something, and they are matter-of-fact and the confidence is not self conscious, then it remains confidence. However, when someone goes too far, they start to sound like they are talking themselves up; advertising; perhaps trying to sell something that isn’t there; that’s when it starts to become arrogance.
Another form of arrogance is not caring about anyone else, and not respecting anyone else. It is when a person does not acknowledge anything good about any competitor.
Confidence has to be reasonable. If you pretend to be confident about things you have no way of knowing, that’s just arrogance. When you are so sure of yourself that you’ll say anything, that’s arrogance.
There’s that saying, “If it’s too good to be true, it probably is.” Well, there is a human analog. If it sounds like too much confidence, then it probably is.