Why do dwellers think they can't carry 100 items?

Solution 1:

Well, I think this may be a bug but there might just be a random number generator somewhere between 60 and 100. So if you send a dewller out into the wasteland then when he or she will come back to the vault when they reach this randomly generated number. If you want to get legendary items then do quests because I heard that in the newest patch, you won't find legendary items in the wasteland. Anyway, completing a quest is always the best way to get rare and legendary items so I suggest upgrading your Overseer's Office to level 3.

Sending Dwellers out is a good option for common and rare items but you don't get as much junk as in quests.

P.S. Maybe you can go through that patch again.