Some Questions about top and SystemMonitor in Fedora

When I first came into contact with Linux, I knew the top Command, and later I found SystemMonitor similar to Windows Task Manager in the graphic interface, however, I found that the two are very different in terms of memory statistics. SEE: Pay attention to used at Mem, and then look at SystemMonitor. We will find that the top Memused and SystemMonitor are very different, later I learned that the Mem of SystemMonitor is

When I first came into contact with Linux, I knew the top Command, and later I found the System Monitor similar to the Windows Task Manager in the graphic interface, however, I found that the two are significantly different in memory statistics. SEE:

Pay attention to used at Mem, and then look at System Monitor.

After careful observation, we will find that the top Mem used differs greatly from the System Monitor, and later we will know that the Mem of the System Monitor is = used-cached. This is true in the computation, let's look back at man top. At the same time, a big brother on shuimu introduced me to me:

The memory meter in htop says a low number, such as 9%, when top shows
Something like 90%! (Or: the MEM % number is low, but the bar looks almost full.
What's going on ?)

The number showed by the memory meter is the total memory used by processes.
The additional available memory is used by the Linux kernel for buffering and
Disk cache, so in total almost the entire memory is in use by the kernel. I
Believe the number displayed by htop is a more meaningful metric of resources
Used
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Htop author's explanation

The two are actually correct, but they are different in terms of memory usage.