✔ 最佳答案
This is for numerous reasons.
The first one is that in an English sub, they are actually just translating. Most likely the person translating knows the original language AND english, so they can better portray how the author wanted the characters to speak like.
In an English Dub, they have to take into consideration about profanity, cussing, and the length of the characters talking. The words will not always be translated exactly like the words in the sub or the actual japanese because it could be a more appropriate version, or a shorter or longer version. It wouldn't be right if they didnt stop talking but their mouths stopped moving.