The system Airbnb uses for trip alterations is really poorly implemented.
Essentially, in the background, it cancels the reservation and re-creates a new one. The problem is if you're using dynamic pricing that has changed the pricing of the already booked dates since the booking, it re-creates a new price for the entire stay based on those lower rates.
Example:
6 months ago a guest booked 5 nights for $300/nt = $1500 total
1 week before their stay, guest wants to extend their stay to 7 nights. But now since it's so close to those dates, your dynamic pricing has updated the price to $240/nt.
Instead of Airbnb adding $240x2 = $480 and charging the guest $480 for the 2 additional nights, it now recalculates the entire stay as...
$240 x 7 nights = $1680, and the guest has already paid $1500 so if they put in an extension request Airbnb will charge them $180 total ($90/nt) for 2 additional nights if I accept it.
Of course, I have the option to deny it and offer it to them at another price but that creates bad will with the customer, who thinks they are being bait and switched. Which is exactly the scenario I have going on right now, with an upset guest that thinks I am trying to pull a fast one on them because of Airbnb's really poor system for handling this.