1st home - water bill assessment query

November2

Hi, apologies if this has been covered already but a look through the threads hasn't helped. My Niece has just moved into her first home, a one bedroom council flat. She phoned the water board and gave them her details as the new tennant, they then asked her all sorts of questions such as how often she would be using the toilet, shower etc so they could work out how much water she would use in a year and give her an estimated bill! This is just £20 short of what I pay in a 3 bedroom house with 3 people! Very confused as I thought bills were property related not people related, shouldn't her bill be what the previous tennant was paying? Haven't had chance to ask the upstairs tennant what he is paying, should it be the same as hers? Can they tell how much water she has used in a year if they didn't take any readings from her property, she doesn't have a meter! Any advice appreciated.