There are a couple of things which could be causing this:
- backlash in the nuts on the Acme screws
- out-of-spec Acme screws — see: https://docs.carbide3d.com/shapeoko-faq/how-to-calibrate-the-machine-for-belt-stretch/ — this should be something of a last resort for a Nomad, or only used for a specific pair of dimensions
- under-dimension endmill
- endmill deflecting — what material are you cutting? see: Adding geometry to cut as a pocket with a finishing pass
- lost steps (not likely, but let’s cover all the bases) —
- inaccurate measurement — what tool are you using to measure?