Reflected light meters all comply with the ISO standard, using a K value in the ISO equation which has a manufacturer chosen value!

N^2/t = LS/K, where 10.6< K < 13.4

Incident light meters all comply with the ISO standard, using a C value in the ISO equation which has a manufacturer chosen value!

N^2/t = ES/C, where 240 < C < 400

The fact that any two randomly chosen meters concur on the same reading is pretty much coincidence!