Ruskie's set me a challenge now, to explain so she understands. (I was a maths teacher in another life!)
If you go out from Richmond Buildings onto Dean Street on 1790's map, on your left is 58 Dean St, next door it has 9, but that means No 59, next door again it has 60, then the next house has 1 but that means No 61. You would have given your address as 61 Dean St, but it would have been shown on the street plan as the No 1 between nos 60 and 70. What an easy to use system - not!

The No 7 Richmonds Buildings could have been bigger than all the others because it could have windows on all four sides.
Ah, and I follow your meaning too, Steve