@ Lemsip,
At a practical level, many E side cables are underground whereas D side cables are a mixture of underground and overhead without screening.
It follows that overhead cables are more susceptible to all sorts of RFI.
EDIT re FTTC cables, note that the phone lines on BT cabinets are Shared (Metallic Path Facility) connections, so it's possible, although less likely, that noise is still being injected from E side phone line cables.
Kind regards,
Walter