Because these materials do not reflect light diffusely, V-Ray computes these illuminance values with very low precision - or it may even skip the illuminance calculations completely. Illuminance values shown over glass and mirror objects represent the illuminance on its surface, not the surfaces that are seen through the glass or in the mirror. When using the same settings (min/ max and scale type) as the VRayLightMeter, the results should be the same for the total illuminance. Simply change the setting and press the Update button. After the render is complete, most VRayLightingAnalysis settings can be adjusted without having to rerender. The VRayLightingAnalysis render element is a post-effect and is only updated once the rendering is complete.