The rate often includes products such as breakfast. When you change the number of guests from 2 to 3 persons, the net rate will remain the same, but the guest should pay for the additional breakfast as an additional person will consume breakfast.
If you still would like to offer the guest the same rate, despite the breakfast, then first change the number of people, and then in the “pricing” tab of the booking, you can set the price back to the original value.