2nd guest and Airbnb charge

[Jessica40]

Hi there

I've had a guest mail me to change their reservation as a second guest would be coming with them. No problem, I don't charge additional rates for a second guest but like to have it on the reservation so I know who/how many are coming.

I've amended the booking to send to them and then on the review page where you see the old vs new reservation, I see that Airbnb has charged me more even though the cost of the reservation has remained the same.

The only difference between the two is that there are two guests instead of one.

Does Airbnb charge more for a second guest?? If so, why wasn't I warned until after I submitted the amend?