The rating system currently goes up to 5.9, as has been pointed out.
It is not a static rating system as in a few years' time almost everything will be at maximum rating and it would have no value in discerning between systems - unless a system's given rating were to be reduced over time, which would not make much sense.
(Kind of in the way games being rated out of 10, or as a percentage is meaningless after a few months.)
The purpose of WinSAT is to provide a single "number" which gives an indication of your system's readiness to run different pieces of software, and manufacturers then just need to give ratings for "required" and "recommended".
It is an "at a glance" single variable which anyone can see, and it does make sense for it to be the lowest of all the sub-ratings as that is your "weakest link"
Most kids go for 3DMark as their e-peen
The idea is that this single number is something any "non-geek" can shop around with, rather than having to be aware that their Ninjamatic x5000GTSLi with DDR6 RAM and their CoreBlimey8 dual hyperthreaded-quadcore CPU with 2TB of cache is sufficient to run Quake 10 - but only if they increase the free disk space from 13MB and replace that MCGA monitor.