When the banding system first came into play, they had assessed my former home as band B when it was the only house on the street with two bedrooms. All the others had three bedrooms. I appealed and a woman came to look at our alleged third bedroom (which was in fact a small 'box room') she took one look and said "you couldn't swing a cat in here, much less a bed." So they reduced our house down to band A.
I reckon the surveyor detailed to assess the houses on our street just stood at the end and wrote down 'Band B' for all of them without actually doing his job properly and looking at the houses.
Slightly different situation, we also bought a house three years ago and it had recently had an extension done. This could have taken it into the next banding but we were just under because the house was low down in the band anyway and the extension took it to the upper limit, but not over.
The system as I have found it was perfectly fair.