free, top and htop show different memory usage
 

I'm trying to meter my memory usage. When I use these different programs, they all show me using different amounts of RAM. Why is this?

I'm just ran all three of them side-by-side and their results seemed to be consistent with each other, although htop seems to display slightly different metrics from free and top.

Consider all of these to be approximations, and consider their numbers as being intended to be viewed in relation to other rows in the outputs of the same program ... not for comparison to different programs.