Z-differences between machined surfaces of equal height in model

I'm machining in plastic stock and a 10mm ball nose (0,39"). I have run both a "parallel" program and a "morphed spiral" program on the same model, and I get similar discrepancies between the model and the machined parts from both programs.

Mainly: The middle area marked in the pictures is lower than the two ends.

Any ideas on what might cause this?

More details; the parts marked U are raised and the parts marked N are lower. The zoom photos are of the part made with spiral passes.

Strange fault. How are you holding the pieces down when cutting them? No chance they are bouncing very slightly?

Sometimes I have solved that kind of issue by doing the same op but in climb cut (or the opposite) afterwards.

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hi :) face and dial the part before begining, then run a few prefinishes and dial the part between them

also, after removing the part from machining fixture, clamp it in another manner, with minimal holding force, and dial it again

like this, you should find if deformation occurs during machining, or after removing it from the fixture

it may be required to change a bit your model, in order to have a flat surface, that you may use to dial on it; is possible to entirely avoid these flat surfaces, and dial directly on a 3d complex shape, but this procedure is a bit more time consuming, and it's precision may be too high for finding the flex source; normally, i use it for surface prelevation of complex shapes, so ....

kindly :)