Its not a slip up.
When your close to the box you should be connected to a 29db attenuated tap port. When your far away you could be connected to the 15db tap. The further away house should in theory have 14db lower upstream than the house closest to the cabinet.
Remember upstream uses a very low frequency. If I test the upstream at the pit using a sunrise meter it may be 30db. Add 50m cable and retest it sits at 32db. The length of the cable doesn't affect upstream that much at all.
A few areas I've been in have a 4 db tap port which now is impossible to get in spec. Either you leave the upstream in the high 20s or the downstream at -6
