It is a combo of those two, @Sha10. The default is one guest on the screen, so the person making the reservation has to manually change it to two. It often gets missed. I have the same price for one person as two, so it is easy for me to ask my guest to change it to the correct number to keep the system happy. They usually do it straight away, and if they cannot find the button, I do it and they always approve it.
Sometimes a booker will be confused and think he is making the reservation, and his girlfriend is the "one guest". That takes a little more explaining 🙂