Once you log in with an account (from not being logged into anything) that account is locked to whatever units you have that first time. It can never be overwritten.
For example.
1. Blank, new game w/o account ==> tutorial ==> roll ==> that roll sucked ==> save to account A ==> logout
2. Since you're logged out, blank, new game w/o account ==> tutorial ==> roll ==> that roll sucked ==> save to account A ==> logout
3. Since you're logged out, blank, new game w/o account ==> tutorial ==> roll ==> that roll sucked ==> save to account A ==> logout
Account A will have the units you rolled in step 1. Step 2 and 3 units are in the ether, gone forever.
Now, say on the fourth reroll you get good units. Now instead of logging into Account A, you log into Account B, and don't logout. You continue to play.