How I explained it was how CMs were explaining it to people they were trying to convince to get in line on our last trip in March, but I didn't explain it very well.
The situation I was thinking of (and where what I said fits) was when the end of the line is close to the sign. If the line extends outside the ride past the line, the wait time is from the sign. That's what I have heard CMs tell people on trips last year in July, last Christmas and Spring break this year. So, I'm sure that hasn't changed.
If there is a bit of space, but you can see the end of the line, the wait will be about what the sign says, but should be a bit shorter because you are starting out closer to the boarding area. That's the situation I was talking about where the wait will be less than what is posted. It happens specifically on the shows or "batch" load rides. If you are waiting for the next show (like Bug's Life), they don't change the sign to indicate the wait until the next show (which could be starting in 5 minutes from the time you entered the line), they usually keep the sign at 15 minutes or whatever is the show time.
The thing I didn't explain well is when the wait is very short. If the sign says 5 minutes and you are entering the line at that time, your wait should be about 5 minutes. You are entering the line at that point - you don't see people ahead of you, so you are the end of the line. That's what I meant. For some things at certain times (like the Safari or Spaceship Earth, late in the afternoon), the posted time may be 5 minutes - which means that you are basically walking right in.