I think this would mainly be due to the fact that 'base' is a dealer created product, the list of possible cuts and impurities are endless. Technically it could be racemic or pseudo meth, so there are more potential synthesis routes which means more potential impurities left from the synth. Add to that, your neighbourhood dealer can step on it with absolutely anything and you can still eat it, cut it with glucose and snort it or IV it. Ice has to be pseudo meth so the synthesis routes are more limited, it also has less viable cuts as the product is generally smoked/vapourised and so the number of agents that can be used to chop it are considerably less.
Recooked perhaps?
For most its still 60 percent, if that.