This is a great writeup on a simple bit of math that violates common sense expectations. Consider you need to go on a 100 mile trip and want to average 100 miles per hour. Conditions are bad and for the first 50 miles you only average 50 miles per hour. How fast do you need to go to average 100 mph for the whole trip? Clearly 150 mph, right? Except of course you are too late and would have to go infinitely fast as you need to complete the trip in one hour and have already taken an hour.

From the article: "But while there’s a lower bound to how “under” the median a step can be – a step can’t take negative time – there’s virtually no upper limit to how much over the median time a project can take."

So when one step takes a little longer than expected most people fall into the 150 mph trap and get frustrated when they don't meet the false expectation.