time flies when your having fun

http://www.scientificamerican.com/podcast/episode.cfm?id=if-time-flew-you-had-fun-09-12-15

They say time flies when your having fun, but a study in the journal Psychological Science they found when one perceives time to go faster they had fun. Subjects were tested listening to a recording, one ground for around five minutes, the other around twenty minutes, both being told the recording was ten minutes. The results were the five minute group had more fun than the twenty minute group.

After reading this I can't say I am quite convinced. What if you are doing something you genuinely enjoy and time does not seem to "fly"? Does this mean you did no have fun? I'll agree that when time moves fast things seem to be more enjoyable, but not in every case. And how does this compare to how we perceive memories where time seemed to stop? I feel like time flew regardless of a good or bad event most of the time when regarding a past memory. Perhaps I just have a bad memory.