I read that - but didn't (and don't) know what they considered their "super-fine" line. They list 400 as both ultra fine and super fine. Their individual descriptions use "ultra fine" for 150 up, and the only place I found "super fine" is in the separate listing in the A4 sheets - that page says they don't always have A4 sheets of 400 and 325, so created the "super fine" category as a grab-bag type purchase.
Did you try just emailing them? Give them your order# and ask if they can tell you what the free sheet was? If it's coarser or finer than 400 then you will know which is which.