However, I noticed back in January that there was one day where there was some 'bursts' of sun that brought the panels up to near max efficiency, approximately 4.3 kWh AC. On February 3rd, I assumed that with an amazingly clear day, with only sun in the sky, I would reach the same 4.3 kWh, but I didn't. The day still turned out to be the best day yet, 18.95kWh.
So I thought about it and can't explain while the max didn't get reached, I could explain how the total energy produced would increase even if the max output at any one point wouldn't be increased. And that is simply that on shorter days, the max output is a very sharp peak where as longer days will have a flat top and a much longer curve to the graph, with no distinct peak. Here is a simple chart that shows this and it is only a difference of 5 or 6 days!
The values you see there are averaged over 5 minutes so you can't actually see the 4.3kWh but you can see that the curve is flattening out at the top with the blue line (Feb 3rd).
It is 8:45AM today and the panels have already reached 1kW/h production which was not reached until 9:45 on February 3rd so if the weather stays the same, you can bet that the 20kWh for the day is attainable today!