Files write to array while cache still has space

[Hastor]

I have the minimum free space on my array set to 150GB. This overrides any setting you have for the cache, or I would have it a bit bigger for the disks to ensure free space, and smaller for the cache.

In any case, my cache min space is set even lower than that, so it isn't a factor here.

 

When copying files to the cache, it will still have 170GB free at times when it switches to writing to the drives in the array. Why doesn't it use the last 20GB always? I'm not using anything other than the share - no VMs, Dockers, etc. On latest version of Unraid etc. Not the biggest deal, but I like to understand what's going on.