Maybe down to the default profile that BT and Sky order. They have a choice of 3 DLM profiles, willing to bet Sky pick Stable which will mean deeper interleaving depth on FTTC.

This is the only DLM control the provider has on FTTC.

I have 2 FTTC lines too, one with plusnet and one with BT. Both modems are connected directly to a Draytek 2920n.

Both lines are fed from the same cable to the house and seem to work ok. I get 45mbit on one and 50 on the other with full upload on both lines.

The lines got installed on the same day, initial speeds were 50/20 on each. The plusnet has dropped slightly where the BT one hasn't. I was 6th on the cabinet, So I expect one line has been a little bit more susceptible to crosstalk. Oddly for gaming my plusnet line is about 10ms fast ping wise to most servers, so it could be different interleaving levels.