it is always the outliers

原文:http://articles.latimes.com/2013/oct/12/autos/la-fi-adv-hy-self-driving-cars-20131013

The challenge for automakers will be programming cars to navigate complicated and unexpected conditions, Sierhuis said. "It is always the outliers — the very complex traffic situations that are hard to imagine and hard to test — that are difficult," he said.

看得懂大意:
"Sierhuis 認為完整的導航功能和突發狀況的處理是一大挑戰, 他說 "預測這些複雜的交通情形是非常困難的"

但我看不懂"It is always the outliers "在這邊是什麼意思