Would you tell me why you use notify_one instead of notify_all here?

[CoinCheung]

https://github.com/mtrebi/thread-pool/blob/99ae49be3289a61c961a45a367873b1e99ddf76e/include/ThreadPool.h#L94

I am reading the code and try to learn from it, and I am puzzled that you use notify_one in this line rather than notify_all, would you please tell me why notify_one is better here ?