The system.swap.size key returns swap usage statistics. Windows doesn't provide swap file information, so the data is calculated from virtual memory (physical + page file) and physical memory statistics. Apparenly the calculation is not so simple and the resulting swap file statistics are approximate.
Additionally it appears monitoring the whole virtual memory statistics is normal on Windows and users expect it also from Zabbix agent. This means it's better to add a new Windows key to allow monitoring of virtual memory statistics directly returned by Windows API.