It looks like the "MemAvailReal" actually only reports free memory, not available memory. Is there a more recent version of this MIB or a different standard MIB which measures available memory?
I tried this from a CentOS 7.3 system with the following packages, but pretty sure this is the same behavior in the most recent point releases as well.
Example of behavior:
[root@hostname ~]# free -m
total used free shared buff/cache available
Mem: 7823 232 4183 156 3407 7097
Swap: 2047 12 2035
There is ~7.0 GB of available memory, however...
[root@hostname ~]# snmpwalk -v 2c -c communitystring 127.0.0.1 memAvailReal
UCD-SNMP-MIB::memAvailReal.0 = INTEGER: 4294500 kB
The "memAvailReal" MIB seems to report the free memory.