How is calculed Memory Usage and how to retreive the daily average

[YellowSky]

Hi everyone,
i wonder how is calculed the memory usage in domoticz.
Indeed, domoticz show 15% and i have this from cat /proc/meminfo
MemTotal: 994232 kB
MemFree: 536836 kB
MemAvailable: 752588 kB
So about 25%
Why there is a difference

Moreover, if i try to get history of this sensor by json (https://www.domoticz.com/wiki/Domoticz_ ... 7s#History) i can't find any way to retreive the mean daily values.
Any idea?
Thank you