I agree, this is the first annoying question to ask. As a seasoned club player, i believe to know that "clean Pawn up" means chess wise roughly what you think i

I agree, this is the first annoying question to ask. As a seasoned club player, i believe to know that "clean Pawn up" means chess wise roughly what you think it means, namely: a) You are in a position, where it really does make sense to count pawns, a so called "quiet" position would be one of those. b) Your opponent has no pawn compensation of any kind, not even in the form of a nuisance. I personally learned this expression from GM Nakamura. Clean pawns were unknown before the "golden era of chess", you could utilize the idiom "ceteris paribus" instead, this was used by the old german vice WM Tarrasch. Hence it should be clear to anyone that without engine assistance it is totally impossible to determine for 10^6 games if a certain material difference was "clean" or not. Most likely he just checked for a stability over n consecutive moves, at least this is the usual way. Others have noted more problems, that might help put his highly deviating, washed out results in order.