Playing on a singleplayer world with multiple characters causes items,locations,xp,(all data) to be kept between both characters. Only on LAN will both characters maintain their separate locations and inventories.

How to recreate:
Start a singleplayer world with Character 1. Get some items, gather some xp, move around.
Close world.
Go on a second minecraft account (Character 2), and join the world you created with Character 1.
Character 2 will now have everything Character 1 had.
Open to LAN.
Both Character 1 and Character 2 with have the same items, xp, location, etc.