System Monitor's History shows high ram usage after playing RDR2

When I start the PC “Plasma System Monitor” usually reports about 2.6 GB RAM usage.
After I played “Red dead redemption 2” (RDR2), System Monitor displays in the “History” tab a RAM usage of between 5 to 6 GB. So more than double, even though no more processes of RDR2 or Steam is running.

Even stranger, I cannot detect any unusual processes with high RAM usage in the “Process tab”.
I also executed ps -eo pid,ppid,uid,cmd,rss,resident,size,share,vsize,drs,vsz,oom,oomadj,trs,vsz --sort=-rss before and after running RDR2 and compared those results. The RAM usage for the processes reported there is around the same (except for a few megabytes).

So is System Monitor’s history tab reporting wrong numbers?
And if not, how can I find out what process is using ~3 GB more before RDR2 was running?

Thanks in advance for any help!
Regards, Marcel