If two accounts are managed on the same browser, account information and transactions from both accounts are liable to be retained in cookies. These are intended to make browsing quicker, by reloading previously viewed pages instead of downloading them.
Unfortunately the browser becomes confused if retaining details of two accounts, and transactions can suddenly appear in the wrong account.
As has been said, the simple answer is to clear cookies befpre changing accounts. Personally, I do this by routinely running ccleaner or Bleachbit in between accounts. This keeps everyhing clean and deletes unwanted clutter.
