Wrong price charged for number of guests

I had a booking for 6 guests for 3 nights 13-16 May 2022

luckily for me the guest contacted me to say his credit card wasn’t accepted and as such his booking was cancelled. He had only been charged £435?! I told him that price was incorrect and he sent me the email confirmation from Booking.com to prove the point.

The booking is cancelled but please could you tell me why it would have been booked at this incorrect price by Booking.com 

Elizabeth Dawe