Probably because BT only served upto 8Mb to you, whilst sky were willing to give you upto 20 or so. Whilst I stated that BTs network was considerably better, over the term, you need be comparing equal products.
Sometimes the odd LLU network could have been better, especially before they grew to a resonable userbase. Sometimes they were serving up newer tech before BT did, resulting in better download speeds. However, I still stand by my statement as theres a lot more to a connection than max download speeds.
In any event it doesn't matter, as I already stated about 9 times I consider it a complete lotto, so we're just giving personal anecdotes.
Short answer, yes and you can't turn it off.
Longer answer is it's automatically set due to your line stats. I believe the more disconnects they detect, the more likely it'll be turned on, but I don't know many fiber users who have it on. People who do stuff like turn their modem off at night will probably be on an interleaving mode. I would definitely be interested in the interleaving policies of either ISP when you take the plunge.
In any event, come April it'd probably make more sense to post on a few forums asking people what they get around your area and with what ISP. Better to get a feeling before signing a year long contract.
I made a graph for my VM:
Compare it to the BT infinity one I stole from someone off cableforums: