Interesting questions, nothing has significantly changed since they were built, there isn't even room for 5 houses with the entry and the old Martin's (Barclays) Bank being on the corner.
I can only think it was an unusual attempt at matching up the numbers with the other side of the road, as the first house above Highgreen is No 12.
Number 11 doesn't seem to appear on the Council Tax Valuation list, are you sure it doesn't start at 13?
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When the debate is lost, slander becomes the tool of the loser. Socrates