Three things to consider along with everything else:
1. Your eyes are logarithmic, and what may seem like a bright overcast day to you may actually be quite dark, and what may seem like dark scattered clouds may be quite bright. You'd need a light meter to know for sure - but you already have one - your solar panels, with an appropriate MPPT converter, will match the light level, along with the angle the sun is landing on them.
2. Solar panels are most sensitive to infrared light. While they'll convert all along the visible spectrum, infrared levels will affect them more than visible light levels. Clouds of different types and densities will have an infrared transparency that doesn't match their visible transparency, so some clouds may allow a lot of visible light through and little infrared, or vice versa. One of the biggest factors is whether the cloud has ice in it or not.
https://opg.optica.org/josa/abstract.cfm?uri=josa-50-9-876 . Again, what your eyes perceive and what the panels receive may be different.
3. Dirty panels affect the power greatly. If, for instance, you have a month of bright sunny days which result in dust covered panels, then a cloudy day with rain that washes the panels, then a cloudy day that seems darker than the previous day you may find that the seemingly lighter cloudy day produced less energy than the one that seems darker.
These are just a few variables that you would have to track to really understand what's going on - there are many more. It's not a simple problem, and generally it's not worth spending a lot of time on unless you believe there's a significant energy loss and you need to find the cause.