Surprise!
We like positive surprises and fear negative ones.
That means (surprisingly) that it's better to have a consistently negative experience than to confront one that's sometimes negative and sometimes neutral. The TSA, for example, would be easier to take if they were always consistently irrational, time-wasting and disrespectful, thus eliminating the risk and replacing it with certainty.
On the other hand, a positive experience that's positive all the time pales in comparison with the experience that's sometimes neutral but often nice. Beyond a baseline of goodness, you're better off awarding a few people a random discount or a bump up in priority than you are making something consistently (and boringly) slightly more pleasant.