There seems to be a large tracking error with SSP also. I note that in the last few months stocks that were recorded as a loss by SSP where gains for me and some that SSP recorded as a gain where a loss for me.
This is strange. We are listing the close price of the session when the trades have happened, so is the offset you see caused by the difference with the intra day price you have in reality? If yes, it should happen for gains or losses that are pretty small.
I would rather expect that your returns are lower then the listed ones as we do not account for the transaction fees...