If I require three items to do some task, but only have two, I would say I have "two out of three", or "2/3".

For whatever reason, /proc/mdstat in Linux reports a degraded RAID-5 array (two working drives, one failed) as having "3/2" drives.

This was super confusing to me, especially since I also had a RAID-1 partition on the same host that had two working drives out of two required and one spare, which I would legitimately describe as "3/2".

I guess as a kernel interface to userspace, the contents of /proc/mdstat are set in stone and can never be changed... but I wish somebody had picked that up when the Linux software RAID feature was being reviewed.