If you log into a world online with a Mojang account, it creates a Player.dat (Player being your username) and if you log in offline it will create another player.dat (player this time being your email address) Here is a link to my world's player foldier https://www.dropbox.com/sh/yrv1z5gzkl6ne59/oQt512LPiS
Not really seeing the problem here. If you log in using offline mode, the system cannot verify that you are the same person, and therefore it's not going to allow you to access the same inventory as if you logged in online.