@Melanie275
You should also have emails confirming each booking with the total for each stay (I hope you have kept these). It's not difficult for Airbnb to see from those/combined with the cancellation policy you had what you should have been paid and what 25% of that equals, so I don't know how they can argue that they can't work this out.
I have not had this problem, but I had the opposite one, where Airbnb tried to overcharge a guest for a cancelled stay. The amount they said she owed somehow changed at some point between her cancellation and what would have been her check in date. It took me a month to get this sorted and even longer for the guest to get refunded the extra money that was wrongly taken from her. So yes, there are weird miscalculations happening sometimes on the system. It's not necessarily an outright attempt to rob you.
I wanted to double check also that you understand that you are not supposed to get 25% of the total booking amount, but 25% of what you would have got from your cancellation policy under normal circumstances, i.e. for a Strict policy, that would typically mean 12.5% of the total booking amount or 0% if it was a Flexible policy. I'm sure you are aware, but just wanted to double check in case that was causing additional confusion with CS.