What sort of device is it? They should be the same.
The RFC Mibs used for this do not take into account virtual memory. The first of the MIBs looks at total physical memory on the device. The second is a combination of total memory used by all the processes running on the device. This value is used in combination with the first to show total memory utilization.
The MIBs used are: HOST-RESOURCES-MIB:hrMemorySize OID: 188.8.131.52.184.108.40.206.2 and HOST-RESOURCES-MIB:hrSWRunPerfMem OID: 220.127.116.11.18.104.22.168.1.1.2.
Because the physical disk listed under the following MIB Table its been listed under the physical disks of the system . Therefor you are going to see the different with Memory used and physical memory
Here is the post you need to check and calculate on each section .