Relative velocity errors in cylindrical coords?

John

I am modelling flow between two co-rotating discs. When I plot Relative X or Y or Z Velocity on the disc surface I get zero as expected (provided I use node values). But if I plot Relative Tangential Velocity or Radial Velocity on the same surface it is not zero! Is this an error?

(I have set the Reference Zone to be the rotating fluid zone.)

Thanks for any help. John