There are a few different reasons siblings could have different father's names listed on their birth acts. You didn't say if the surname was the same so here is what I can think of....
1.) is simply because the father's middle or "sopranome" [nickname]name was used instead of his first name [this also can apply with the mother's given names]. This generally happened when the father named a son after himself and was used to differentiate father and son.
2.) The father has passed away and the mother remarried the brother of her deceased spouse. This would still mean the siblings carried the same surname but the father's were brothers. You would
Hope this helps!