Z values are measurement of how many standard deviations something is from the mean. 95% of your values fall between -2 and +2. Most "interesting cases" are about outliers, something that's very uncommon. If it's common, you don't tend to write a paper about it. Nobody cares if someone had a slightly above average tumor, but if they have 50kg tumor, that's publishable.

The survivorship bias plane shows a world war 2 chart of where the bullet holes were on planes. The conclusion famously isn't to armor those parts often hit, but to armor the parts that weren't hit, because no planes hit there returned to be recorded.