You have two curves slightly out of plane. Depending on how your Rhino Units are set its hard to see.
A quick fix in Rhino is to use the SetPt command and use Z only. Setting your Cplane (set C-Plane by Z-Axis) to the exact Z you are looking for (getting rid of the float)
There are a few ways to get things planar in grasshopper as well, but to ensure the Z is correct I would do the fix in rhino.