Blue buff stolen by nothing

So today I was playing jungle (lee sin). my teammates helped me but when i killed the blue buff, varus got the buff, gold and xp. this was wierd because he did not attack it at that moment, he even wasn't close at all (riverbush red side). I watched the replay and " I " killed it with a q. it landed while it was at 155 hp while a level 1 lee sin q with the build and runes i had would have done 159 damage. so why did i not get the kill but varus did, while i landed the blow that got the camp under 0 hit points?

ps. i dont know how to add the highlight, i would love to add it if you guys tell me how to.