From the documentation:
"pavailable - inactive + cached + free memory in relation to 'total'."

Actual values:
C:\zabbix\tmp>zabbix_agentd.exe -t vm.memory.size[available]
vm.memory.size[available] [u|293146624]

C:\zabbix\tmp>zabbix_agentd.exe -t vm.memory.size[pavailable]
vm.memory.size[pavailable] [d|6.755881]

C:\zabbix\tmp>zabbix_agentd.exe -t vm.memory.size[cached]
vm.memory.size[cached] [u|3863650304]

C:\zabbix\tmp>zabbix_agentd.exe -t vm.memory.size[inactive]
vm.memory.size[inactive] [m|ZBX_NOTSUPPORTED]

C:\zabbix\tmp>zabbix_agentd.exe -t vm.memory.size[free]
vm.memory.size[free] [u|285749248]

C:\zabbix\tmp>zabbix_agentd.exe -t vm.memory.size[total]
vm.memory.size[total] [u|4292448256]

C:\zabbix\tmp>zabbix_agentd.exe -V
Zabbix Agent Win64 (service) v2.2.5 (revision 47406) (17 July 2014)
Compilation time: Jul 17 2014 16:15:35

(0+3863650304+285749248)/4292448256*100=96.67, but pavailable report "6.755881".