In game ashe bug

I noticed while playing ashe that according to her passive every auto after the first should do more dmg but the opposite always happens I saw the numbers drop from 180 to 173 to 166 and so on randomly but with a minimum dmg number which looks to be random as well but based on how much ad she has ofc Was it intended to be like that? I mean ashe is not a normal adc she counts on her Q and passive to make dmg so she is more of an over time dmg adc not burst like some one like a draven or vayne who can 1 Q almost any adc if full build so I wish to fix this if it is a bug and if not consider changing the way it works Cuz the numbers fall dramatically in late game for some reason and again it's always like that not 1 or 2 times per game which accually makes CSing with her pretty challenging some times and thank u