Something wrong with C2?

Now my Total System Equity is 208204. My system started on 4/6/2011 and Apr return is 2.6%. The Total System Equity at the end of Apr should be 102600. Then the return for May should be (208204-102600)/102600=102.93%. Why does C2 shows 99.6%?

The math is correct. The marked-to-market equity value of your system at the end of april was 103,401.

Then, why the return of Apr is 2.6% instead of 3.4%? Even you use 103401, the return of May should be more than 100%, right?

C2 also factors into the returns your system subscription price and the commissions for whatever broker is selected in your "commission plan." You can adjust the commission plan on screen and you will see your returns adjust slightly too.