Electrochemical cells sources of error?

Using an electrochemical cell with the reaction Zn(s) + Cu2+(aq) → Cu(s) + Zn2+(aq), voltages for cell potential were calculated at 1 M for both Cu2+ and Zn2+, then lowering [Cu2+(aq)] to .1 M and keeping Zn2+ at 1 M and then lowering [Zn2+(aq)] to .1 M and keeping 1 M Cu2+. These results were lower than actual values. What would cause this?