The 93% score is taken from all samples, including both ItW and ZOO samples. ZOO samples are those that you're hardly going to ever see - they usually exist in virus labs only (never really made to the wild) (btw the same applies to biological viruses - only a small fraction of them is in the wild [but in this case, there are also other factors such as geographical location etc]).
Avast detects 100% ItW viruses, and a large number of ZOO viruses. The point is, you don't really have to care about ZOO viruses - they're here basically just to make antivirus testing more interesting...
BTW Andreas (the owner of that site) sometimes steps by at this forum, so maybe he could explain his testing methodology in a bit more detail...
Thanks
Vlk