Optimum Levels are based on unit of fuel per distance, Lb/Nam, it doesnt take into account the present winds temps etc, they are only accounted for when you use ECON, Cost Index.
You are seeing discrepancies due to differences in residual rates of climb and buffet margins. If your CFP is always lower, then the company issued parameters are more conservative than the FMS.
Both the CFP and FMS are right, but just different, its up to you to decide if you want the additional conservatism. You could start by asking your CFP provider about the input parameters.
Mutt