I use the field calculator to measure the length of LineStrings in my shapefile layer, but i get different results when using QGIS 2.18 or QGIS 3.0.

If I am using the local coordinate system the difference between the QGIS 2.18 and QGIS 3.0 is not significant. If I am using WGS84 (I know you should always use the local coordinate system) I get two different results based on the version I am using. In 10 km the difference is almost 2 km..

Does anyone know what might be the reason? Did the algorithm behind the length calculations change in QGIS 3?