"Other" includes distribution, automatic notifications (interestingly, the letters for password recovery don't go to that often).
In the "Sorted" get letters from people.
Well, you can tell what letters where to put. Here I received a letter mailing Tinkoff-log and fell into the "Other". You can ask Outlook for the future to put these letters in the "Sorted":
And, you can disable sorting — View → Show sorted: