Just like English,two negatives resolving to a positive.
For example:
“我不是不同意I do not disagree” could mean “我同意 I agree(not mean I quite agree)"。
这不是不可能。It's not impossible,could mean 这是可能的 It's possible(does not mean that there is a great possibility.)
他们不得不离开这,,could mean 他们一定得(děi)离开这(It is not their will)
It is my understanding,I hope to help you.