The sum of vm.memory.size[used] and vm.memory.size[available] does not necessarily equal total. For instance, on FreeBSD active, inactive, wired, cached memories are considered used, because they store some useful information.
At the same time inactive, cached, free memories are considered available, because these kinds of memories can be given instantly to processes that request more memory.
So inactive memory is both used and available simultaneously. Because of this, item vm.memory.size[used] is designed for informational purposes only, while item vm.memory.size[available] is designed to be used in triggers.
See the "See also" section at the bottom of this page to find more detailed information about memory calculation in different OS.