Probably for the same reason that my telephone number which I have had for the last 30 odd years has ceased to be listed in the phone book for the last few years. When I enquired I was told it was ex directory even though neither I nor my husband had asked for it to be taken out!
I agree. The whole of one side of our road, 20 houses have the same postcode - think the opposite side of road (odd numbers) is different. It's not house specific.
My house number is 460 but it does not exist, it is only listed as 458-460. Next door is a shop with a flat above it, and they should be listed as 458a and 458b.
This is causing problems when we register with gas and electricity suppliers etc