Yeah I did the same type test with one neighbor's Canadian Solar bifacial panel my little panel tester, and I was surprised how much output I got when the panel was face down against the ground.
I am thinking that the panel is also semi-transparent so some of the light is getting through and making output from all the cells (without me really knowing how these substrates or cells are layered together), that was my initial thought on it...
I did my test in August around high noon, and I was playing with all sorts of orientations and amazed at how the panel really performed well even vertically, or any angel really. Of course the Sun was very strong (straight above almost), so little angle through atmosphere.
I've tested other panels (not bifacial) at other hours of the day, and they typically they all do much better in midday where angle doesn't matter so much.. The best tests I've done where angle matters more are in morning or evening angles (obviously), which would also put Winter angles into that bucket.
Anyways, thanks for sharing, it is fun playing with panel testing...
