I have just been made aware that the implemented in Icinga2 script check_mem.pl wrong values.
The script is implemented in the following file: /usr/share/icinga2/include/plugins-contrib.d/operating-system.conf
As an alternative, I once included the check_linux_memory script from https://github.com/hugme/Nag_checks and the values match.
Does anyone happen to have the direct link to download the check_mem.pl script.
Thank you for your help