Why is the mass difference in the ud-quark-family different from the other families?

Short answer: There isn’t any known reason.

Longer answer: The standard model of particle physics doesn’t involve any predictions for the flavour structure that we observe such as the quark masses that you are describing. While certain models of beyond-the-standard-model physics might yield specific predictions, as far as I know there is no generic or deeply satisfying answer proposed to explain it.

It might be worth mentioning that the fact that the up quark is lighter than the down quark is very important to us in our day-to-day lives. This leads the the fact that the proton is the lightest baryon, and so Hydrogen is stable. If this were not the case, you can bet we wouldn’t be here today; there would just be a lot of neutrons lying around.