Surfer's contour map and grid editor use different contouring algorithms because contour maps must allow for filled contours and inline contour labels.
The two algorithms can handle saddle conditions differently, which can result in minor differences but both are correct. For example, the Grid Editor connects the lows and the contour map does not. Conversely, you may see that the highs are broken apart in the Grid Editor and are connected in the contour map. Both ways are mathematically correct, although you might prefer to connect the lows in a contour map when they represent a drainage.
Updated November 2021
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