Sometimes, when cats are taken from their mother at a young age, they never learn how to "control" themselves during play. See, when they are kittens they wrestle. They kick, swat, and bite. If one kitten bites too hard, the other kitten will cry out, and most likely quit the game. The first kitten learns that if he doesn't play nice with his siblings, no one is going to play with him at all. I don't think he would learn to do such hard biting with your boxer, for it would still certainly hurt pretty bad. I think the boxer would have let him know, yelping or whining when he bit that hard.
Your cat may have never been taught these restrictions, but don't worry! There are a few things you can do try and fix this. If and when the cat bites you, cry out! Say "Owww!" or something in a high pitched whine, and then walk away.
If you were playing with the cat, that's it, game over. If you were petting him, stop. Give him the cold shoulder for a while.
When you are petting the cat they will sometimes get irritated with your petting and decide they have had enough. Most cats walk away, but some cats will bite to give you this message. Pay close attention- has the cat stopped purring suddenly? Do they suddenly look/feel tense? Sometimes they'll start getting an irritated twitch in the tail. The ears may just slightly
tilt back. Sometimes the pupils will dilate (get biggger) a little. If you notice the signs, just calmly stop petting the cat and let him know she is free to go (if sitting on your lap). Or maybe you have to leave, if your cat wants the bed/couch to himself.