Hi,
we are migrating to ArcGIS Maps SDK for .NET, this includes a GeoProcessing Packages with an underlying python script that uses a call to arcpy.sa.Spline.
We noticed a difference in results on the edges of the result of the spline method.
We use arcpy.sa.spline with a mask applied through arcpy.env.mask and use the following options:
Even though you are mixing languages, it's still exclusive to the language's documentation so you should be able to find mask or Spline information in the arcpy docs. Why it's producing different results could be due to rounding or available memory maybe and probably is worth reaching out to esri support for the issue.