I am working on a plastic matched automotive black (GM 848) where the color is visually darker, but it measures LIGHTER than the standard. Can you please explain why?

It's difficult to say without seeing the samples, but two possibilities come to mind. One is simply that there is some sort of problem with the measurements or the instrument. Assuming that is not the case (e.g., both samples measured at the same time, on the same instrument, in the same way), then I would suspect that there is a difference in the surface characteristics of the two samples. For example, the one that looks darker might have more of a glossy (or less matte) surface than the one that looks lighter. Depending on the instrument geometry you use, this difference might not show up in the measurements. If you take a glossy and matte surface that measure the same for total reflectance (0/d), the matte surface will usually look lighter. If you measure the two on a 0/45 instrument, the reading for the matte surface will be higher, more in line with the visual assessment.  more