Memory usage will within reason always increase as time goes on as more things are cached just in case you need it. An dumped from memory if heavy usage occurs and the system needs to free up that space.
You can not compare data provided by different programs, they sum up information in different ways.
Different units of measurement such as gb vs gib (often not clear which measurement they're using), what data they include in their calculation, if they round up, etc. Have a look at how memory is defined: cat /proc/meminfo
On my system with 32gb ram, which is is not idle and has an uptime of 4 days so large amount of cached data. Used memory is reported as.
free -h
Reports 5.7Gi
htop
Reports 6.18G (6.18Gb = 5.75GiB)
KSysGuard
Reports 5.5Gi
I'm not going to manually add up the 427 processes I have running to see if it even gets close. Reboot and its down to 2gb
In addition gnomes system monitor only shows your users processes by default, not processes being run as root etc. I would also point out that memory is useless unless its being used. I really wish people would stop freaking out over it.
Regardless, you have no problem. But if you wanna drive your self crazy over it and install another distro, go for it.