There's a guy in my 3U class who does 4U maths (and the only person who does).
My head teacher said to the 4U maths guy,
"because you do maths extension 1 and 2 they are counted as two units each and you can't do more than four units of maths, so your 2 unit results wont be counted"
... so there you have it, even though you can sit for the 2U maths exam, your results for 2U wont be counted ... only your maths ext1 & ext2 will be counted ...
and he created confusion in the recordings of the 2unit maths ranking because he shouldnt even be there but he did.