Hello,
Mini update:
This is because in forward, lerped skylight blending needs to be enabled per material (using Blend Sky Light Cubemaps) since it adds a bit of cost and might not be worth it for non specular material. At least that is how I understand the history of it.
In 5.4, this has been updated and you can simply override all your forward material to feature blending of skyboxes for total specular correctness. A decision you can take per platform for a project. This can be enabled by setting r.ForwardShading.ForceSkyLightCubemapBlending to 1.
The change list one can integrate to get that behavior is this one.