There are 15 houses here that share the same postcode (7 in our terrace) and in the same road, my mates garage seems to have it's own postcode.
I'm pretty sure he's never had 20 letters a day though, even including the junkmail!
The block of flats daughter lives in has one postcode per block.
If they sort letters in the sorting office into boxes that are by postcode (or blocks of postcodes) it makes sense they have a similar number of letters in each box.
Cheers., T i m