Why is there a discrepancy in PLAXIS and SPOOKS results for stability analysis of retaining wall?

I have done a PLAXIS 2d model and a Spooks model for analysis of retaining wall. I have included same input parameters for soil properties, loading, partial factors, water head etc...
What I found was the moment in the wall was comparable. However, the anchor loads in the PLAXIS 2d model is substantially larger than the spooks results.
Does anyone have a reasonable explanation for this variation?