In case anyone cares I did some further research and found the answer in the AE bible “Creating Motion GRFX with AE” by the Meyers.
“Whether you set the end keyframes manually or use the write on mode, the stroke itself will retain all the inherent timing nuances (speed ups and slow downs) or your hands movements”.
Hopefully Adobe will correct this issue in future editions of AE if they havent already (I’m using 6.5).