Re: Only 15 years late...
I'm, assuming your FTTN is what we in the UK call FTTC - a VDSL2 service,
AIUI, this may well be down to interference between subscribers to the cabinet. The high frequency signals aren't perfectly contained by the low spec twisted pair cabling, so when several subscribers take the service, and their pairs are in the same (or adjacent) cable, then the signals interfere and the result is a reduction in the number of usable bins or bits/bin for certain bins. The result is that the first person on a cabinet gets a grest speed, but it starts dropping off a bit as other subscribers get connected and the signals start interfering.
Another factor may be backhaul dependent, but with gigabit fibre back to the exchange, you'd need something in the order of 20 users all pulling 50M before you saturated it. I don't know if larger cabinets have multiple backhaul channels (eg 2 or more 1G links aggregated) - but if not, then by the time you get up to cabinets serving hundreds of users, there's scope for local contention for the backhaul.