No youre not missing anything obvious - this is a very common problem experienced by many.
The ISP asks BT to turn interleaving off.. bt says they do it.. but it never does seem to happen.
Ive seen this happen so many times and with all ISPs Im afraid.
The problem you have now is convincing your ISP that you still have interleaving turned on.. and then your ISP has to battle with BT to convince them.
BT will insist its turned off despite you seeing evidence to the contrary... and Ive seen ISPs literally tearing their hair out over this
The best thing you can do is provide your ISP with a screen cap from your router showing that you are "Interleaved" and not on "FAST".
It may also be worthwhile reminding your ISP that if they do a
WOOSH test on your line then it should show them that you are on interleaved as well.
Good luck.