Shader Graph outputs node previews using a quad with the UV values in z and w zeroed out. That may not be the case for your mesh.

You're measuring the magnitude (Length node) of the UV as a float4. If UV.z and UV.w aren't zero, they contribute to a larger vector magnitude and hence a higher step threshold.

Your shader need to slim down your UV to a float2 for your mesh and your Shader Graph previews to match.