Everyone seems to be overlooking the obvious regarding RiscPC's - they are pretty much the oldest machines that are usable today (i.e. can run RO 3.6-4.39 at a reasonable speed).
They sold very well back in the Acorn days, and people won't have chucked them out, or else people would have bought them cheap 2nd hand. I expect there were many more RiscPC's sold than Iyonixes, so what's the surprise? Iyonixes are new, but expensive and don't have 100% compatibility.
I'm also not at all surprised that VARPC has sold more than the Iyonix - it can run Select, it's generally faster, you can have it in laptop form and it only costs 120ukp (I expect most of us already have a suitable Windows PC).