Multiple accounts on one computer share mist energy to prevent system abuse by using multiple accounts - if you go down to 10 mist on one account, then swap to another, that account will also have 10 mist.

From your case I can guess that if you don't use that 10 mist, the system recognises that and shows how much mist the account should have if you log on at another computer. Chances are if you touched that mist before switching comps it'd lock in, though.

For the record, accounts which have at any point bought energy using cash are exempt from the shared mist rule.