You have 100 items, 5 are zero, 5 are 100 and the rest are 50. Your average is 50, and 90% of your items are average. These are your average items (the 90%)

In this case, though, average is meant in its colloquial form of another way of saying "normal".

For 2017, the average household income was $89k, which corresponds to 65th percentile [0].

There are many "us vs. them" thresholds to choose from; curious what was special about this one.

[0] https://dqydj.com/2018-average-median-top-household-income-p...