My understanding of how this would work is:
During the month of May, you conduct your transactions (if any) and get charged fees accordingly.
At the end of the month of May, if Total_of_Fees < $25 then they charge you $25 - Total_of_Fees .
Of course, this would be logical, and nothing I've seen so far has been logical!!!
By the way, as of 05/03 morning, I have a pending charge of a few cents on my accounts even though I have conducted no transactions this month. I'm keeping a close eye on that. It may be some leftover fees from last month.