Yeah, as @MandyGeorge said, this is normally the "grand total" for the metric.
Basically, looking at non-averages, you might see:
Which is kind of useless...
Now, if I filter my table:
Then my total will reduce, but still show the original grand total:
Averages, while not a "grand total" will still show the unfiltered average.
You can hide the "out of" per column by deselecting Show Grand Total:
Or, you can shut this off for all columns by default by setting up your preferences... I do this, I really don't need to show the grand total for most uses, and if I do, I can turn it on for the rare times I want it.