It's because you have already reached the maximum level of how much RAM you can clean. The reason why it started at 2.9gb and then 3gb is probably because it was so close to 3gb before. The killed apps that are required to run might collect some information when they start again.
My personal recommendation is stop using RAM cleaners. We have this one integrated because some users insists on it but there's really no need considering how android works. I've never cleaned my ZenFone 6 and it's as fast today as it was when it was brand new.
