Running R 4.1.0 in the RStudio IDE on windows w/16 GB RAM & I noticed that the memory usage report in RStudio doesn't match the task manager. EG, with a new project & nothing loaded, the memory usage report says I've got 49% of memory in use, but only 125 MB used by R/Rstudio. The task scheduler says RStudio is using 700 MB of memory, but matches the 49% total.
The memory manager tracks what's being used by the R session itself ("used by session"), which is a subset of the total used by RStudio. The Task Manager is also adding in what is used by RStudio's GUI and front end.
If you expand the "RStudio" item in the Task Manager, you should see an "rsession" sub process whose memory usage matches exactly with what RStudio's reporting.