Detailed memory usage value seems incorrect


My question is about the values presented in the view “Detailed Memory Usage” and specifically the value reported as “-Sh,Bu,Ca”.

My guess is that the value is supposed to represent the amount of memory that is used (RAM Used from the line above) minus the memory classified as Shared, Buffers and Cached. However, looking at the RRDTool Command by pressing the “Show RRD Command” link suggests that the value is calculated like the following:
If I understand the syntax correctly, this is equivalent to usedreal-(cached-(buffered-shared)) instead of usedreal-cached-buffered-shared that I was expecting.

I’m not sure if this is a bug of if I am just misunderstanding what the value is supposed to represent.

Version	1.48.1-25-g21192a9cf - Fri Feb 08 2019 15:17:29 GMT+0100
DB Schema	2019_01_16_195644_add_vrf_id_and_bgpLocalAs (131)
Web Server	Apache/2.4.25 (Debian)
PHP	7.0.30-0+deb9u1
MySQL	10.1.26-MariaDB-0+deb9u1
RRDtool	1.6.0