A list/segment has total number of users present in the respective list/segment. However, when the same list/segment is selected in APN broadcast, it gives a respective reachable count. Now this count is the count of valid app tokens mapped against the users present in the list/segment.
It is not mandatory that all the users present in the list/segment will have app token. Only the users who have it will be considered in reachable user count. One single user can have multiple tokens if the same id is used to login in multiple devices.
So the 2 cases which might cause this difference of counts are -
1) One user having multiple tokens. For example if a user has 2 tokens, then in segment the user count will be 1 whereas under reachable users the count will be 2.
2) No app token mapped against the user. In this case, under segment the user count will be 1 whereas under reachable users the count will be 0.