Why is it obligatory to make up (the fast) in the first case (breaking the fast while doubting the sunset, and it becomes clear that the sun had not set), but not obligatory in the second case (eating or drinking while doubting the dawn)?

A fasting person must make up the fast if he breaks it while doubting the sunset, because the default is that the day remains, and he should not break his fast except with certainty or a strong presumption of sunset.

However, a fasting person does not have to make up the fast if he eats or drinks while doubting the dawn, because the default is that the night remains, and eating is not forbidden until dawn becomes clear.