I did the same test recently , a combro through my f1 beta and the combro was lower, i trust my f1 because it matches the skan at the club,
Thats the second combro i've come across that reads low, only 30-40 fps but if someone sets there gun at 11.5 it could actually be more!
Trouble is not everyone can afford a high end chrono and the combros are very popular, better to have one rather than nothing i agree, but unless you compare readings how will you know for sure?
Anyway back to the F1, works better in the shade even with the diffusers on, i was looking at some 12v led's so i can set it up in the shed, work in progress when i get round to it