Used to be called the Match of the Day effect.
Fans watch their own match and see a fairly standard 0 goals from set pieces scored. They then go and watch MotD and over the course of 9 matches see a few set piece goals. Fans brains do their usual pattern spotting and extrapolation (as they're evolved to do) and fans get angry that their own team doesn't score enough set pieces.
It's true. I like to read the Liverpool forum RAWK (red and white kop) and on there, they are always saying how phucking useless they are at corners -- yet by goals scored from them, they are currently one of the better sides. And they don't even have that many players who are good in the air.