Because the wait was how long it took someone who entered the line 10 minutes ago. The wait time is mostly updated off the FLIK cards they hand people as they enter the line. So when Person A gets a card and gets in line, their timer starts. Now Person A only takes 10 minutes a there is no one in front of them, but between then and Person B, everyone else (like 50 tour groups) finished up their breakfast and got in line. So Person B is still in line with the next card when you show up and see "Hey! 10 Minute wait". Shortly after you get in line Person B finally gets to load having waiting 40 minutes, and the time should update while you are in line.
It's kind of like a traffic report. You see that it's reported to take 30 minutes to get to work, so you leave at 8:30. But everyone else saw that as well, so they hop on the roads, and an accident occurred on your route. So your commute now takes 45 minutes because of the extra people on the road and the accident.
They may inflate the posted wait times for more popular attractions at the end of the night to keep people from getting in line (CMs can't leave til everyone in through) but they gain nothing from under reporting the wait time.