Steps to reproduce:
Request key vfs.fs.get from Zabbix agent 1 and Zabbix agent 2 on Windows

Result:
Agent 1 reports lpFreeBytesAvailableToCaller parameter received from GetDiskFreeSpaceEx() as free bytes value.
Agent 2 reports lpTotalNumberOfFreeBytes parameter received from GetDiskFreeSpaceEx() as free bytes value.

In order to reproduce the issue, a system where these values are actually different is required.

Expected:
Zabbix agent 2 should be adjusted so that both agents report the same value as free bytes value.
As explained by sasha:

It appears that the first Windows agent behaves consistently with other platforms. Therefore, the behavior of the second agent needs to be corrected.