In Surfer 14 and newer, when you use the Grids | New Grid | Grid from Server command, or click the Download button in the Open Grid dialog to download a grid from a WCS server, the Z values in the downloaded grid are the true elevation values. If you choose to download a grid from an OSM or WMS server, the Z values in the downloaded grid range from 0 to 1, because these are not true elevation values, but are values assigned by Surfer to represent the colors of the pixels in the downloaded image.
Since Surfer 13 did not yet support WCS servers, downloaded grids showed z values in the 0-1 range. Although these are not true elevation values, the grids can still be useful to get an idea for the relative elevation of the area. Note: if you will be downloading grids from a WMS server, you will want to choose a grayscale terrain layer like the ones in the Terrain Data category, since the color in the image reflects the variable that you want the Z value to represent. If you choose a layer that contains something like an aerial image, the colors in the image do not reflect the elevation or any other value.
Updated August 31, 2020