It seems counter-intuitive, yet on a cloudless, sunny day, our panels peaked at a lower electricity output than on a day with overcast. In fact, on the cloudy day, our panels maxed out the inverter, causing it to clip. See the graph below where I combined the charts of 2 different days.
The red line shows the nice production curve on a cloudless day, but the green line shows the output on a cloudy day. The green line maxed out earlier in the day and even later in the day than the red line.
The sunlight hitting earth must be reflecting back up to the sky, but the overcast is causing it to reflect back down again. This results in more solar energy hitting the panels. I would assume that the glass over the panels is like this as well - allowing the sunlight to pass, and some light that reflects off the panels would get reflected back to the panels by the glass.