I find this odd, when I tested the lens I found 70mm to be the sharpest focal length on the lens. It does seem to be much sharper at f/3.2 than f/2.8 though.
I don't always trust the Digital Pictures chart tests, there is more to a lens's performance than a test chart. Some of their Zeiss charts look horrible but when you use them in real-life situations they look absolutely incredible. I will say that their tests on longer lenses seem to be more accurate than their tests on shorter ones.