most likely what the bank is saying is that your offer was accepted subject to the final approval, possibly from the asset manager's contact at the bank, or the bank's manager or some other 3rd party.
so it sounds like what happened is while they were trying to get the final approval on your offer, another offer came in that they felt was a better offer (higher price, no financing, etc)
so since your offer still had a contingency (the 3rd party approval) when the other offer came in, they took the better offer
yen offer isn't a contract till it's approved
take a close look at the language in the seller's addendum and that may shed some light on it