The value in the computed length field depends on the map projection used in your canvas. Web Mercator is useless for planar measurement (the poles are infinitely far from the Equator, so all measurements not along the Equator itself are skewed, potentially infinitely), as are all geographic coordinate systems (Cartesian degrees are meaningless).
Some measurement calculations in the UI are smart enough to not trust Web Mercator, and to do geodesic computation with a geographic coordsys, but the table rows are going to return Cartesian results (the units of which may not make sense).
If you use a coordinate reference which is not the same as the data source, any stored values in the original coordref may not agree with the values computed "on-the-fly" by reprojection. The amount of error may vary by location (e.g. over Africa vs. over Iceland).
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