Waits times for most attractions are just slightly posted above the actual wait time to account for a minor stop, to make Guests feel better, and to round up. Most attractions with a Fastpass Merge Point have the "actual standby wait" posted at the bottom of their screen now. It also depends on the attractions, the time of day, or if they just had a downtime. Big name attractions like Soarin, Toy Story, and Space Mountain are notorious for inflating wait times to make a truly long wait much more of a surprise that it really wasn't that long. Wait times also jump up after parade & fireworks dumps and following downtimes as they are expecting an influx of people, especially through the Fastpass lines that will result in having to follow stricter ratios sending standby guests less often. Wait times will also be manually inflated to detour people sometimes right after a downtime or closer to park closing. I've seen them do this at Toy Story to keep the line down so that it take forever to clear the attraction. The times when they are not accurate are when you enter and then a downtime, stop, or show/parade dump occurs while you are in line. Then you will typically wait longer than the posted time.
In general though, those wait times have some accuracy to them based on the FLIK cards that are sent through. They won't just make a wait time 50 mins when it is really 20 just because, there is usually a reason. Most of the times waits are something like 43 minutes actually and 55 mins posted or 7 mins actually and 15mins posted.