Yes, I know it seems goofy but that is the way it is. The bank you are dealing with has indicated that they would like to accept your offer, but they also have to check with the company which provided the insurance (pmi) on the loan to the origonal owner. That pmi guaranteed the bank some level of protection against the situation in which they are now faced - that is taking an offer that will not pay off all the debt and accrued interest and fees on the property. The insurance (pmi) company also has a say in the acceptance of a contract and its terms since they also have a financial interest/obligation in this property.
Does this help your understanding at all? (The last one of these I did it took the pmi company about 2+ weeks to give us an answer.) Good luck to you - sounds like you are 3/4 of the way to a new home!