Disk Usage - Something isnt adding up

I recently was looking into the shard distribution in my Elasticsearch cluster, and I noticed a heavy weighting on half of my cluster (some nodes have barely any shards on them). When looking into the distribution what I noticed is that a large number of my nodes have full disks. What is interesting (and the basis for my question) is that when running the following request

GET /_cat/allocation?v&pretty&s=disk.indices:asc

I get several nodes that look like the following

shards disk.indices disk.used disk.avail disk.total disk.percent host            ip           node
27       79.6gb     753gb    117.6gb    870.7gb           86 ip-172-31-3-243 172.31.3.243 es-data-41

What we see here is that there is roughly 80Gb of "disk.indices", yet 753Gb of disk used. While investigating where the remaining 670Gb of data went, the machine reports approximately 709Gb of disk space used in the /nodes/0/indices directory. To my knowledge this is the data store for indices (the one that is reporting 79Gb of space). What gives? What am I misunderstanding?

I think I figured out whats going on. We had some instability in our cluster, and a couple nodes restarted. "Shards that are unassigned do not count towards this number". Well we have quite a few unassigned shards.... Thanks and sorry for the post

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