SNMP memory free much lower than what's shown on dashboard

[poodad]

When I get the amount of memory present using an SNMP get of 1.3.6.1.4.1.2021.4.5.0, it agrees with what the dashboard shows. However, getting the amount of free memory (1.3.6.1.4.1.2021.4.6.0), show much less free memory than what the dashboard shows.

Which is correct? Where does the dashboard get its memory stats from?

Is there a better OID for getting the free memory?