hi--not sure why this happens either...but here's good news, maybe:
i had the same experience this week when i added free nights to my guests who had evacuated from hurricane matthew. I couldn't understand why my service fee effectively jumped from 3% to 4.3% even though the dollar amount for the reservation was exactly the same. The only thing I changed was the length of stay. I was about to send a question to Airbnb about this, and then realized that it actually all sorted out....by the time the guest had accepted the reservation stay, the service fee had corrected back to the 3%. It showed up wrong at the outset, but then somehow "caught up to itself". I wonder if this will happen for you.