Map scales are never constant: they vary with location and even can change with orientation. The nominal WebMercator scale applies only at the Equator.
Inherent distortion in the Mercator projection must increase the scale uniformly in all directions as a function of the latitude. In France, it will expand all distances by about 50% or so, varying a little from south to north.
The distortion factor (for a spherical model of the earth) is the secant of the latitude. When 153 meters on the earth appear to be 226 meters, for instance, this indicates the secant of the latitude is 226/153. That corresponds to a latitude of 47.4 degrees (north or south). In the North, that would place you right in the middle of France (near Tours).
Any projection designed for a small country (spanning perhaps ten degrees in any direction) will have relatively little distortion. We may take the map in the Lambert projection to be accurate. Thus, we should expect the Mercator map to show features about 50% larger than they will appear on the Lambert map.