Wanderer loot caps at 100 items (about 4d:4h into the wastes, since there's roughly an item event per hour).
Early on we though that dwellers replaced items if they found a better one (dwellers with their inventory full of blues); this doesn't seem to be the case anymore.
Instead that seems to be the behavior on games that left dwellers wandering for extended periods of time (10+ days) withouth connecting, so they force the game to calculate what happened during those days.
Seems that, Instead of counting from "day0", game calculates loot based on the current day rare chance (rare chance keeps increasing steadily until it caps at 100%, a wanderer left alone for, let's say, 2 weeks without loading the vault, will force the the game to calculate what happened thinking the wanderer is at day 14, with probably 100% rare chance),