In our experience, the posted wait times have always been most inflated in the first hour or so because lines are still growing from the first wave of entrants, and near closing as they try to deter more people from entering the line. In the middle of the day they tend to be more accurate, but are still usually inflated by 10-15 minutes, as stated by the PP who worked in attractions.
I would also think that the existence of FPs makes predicting standby waits tougher. The wait can be estimated by the size of the line, but if a disproportionate number of people suddenly show up with FPs the standby line will slow down. Conversely, if hardly anyone comes with FPs for 10 minutes or so, the standby line will move more quickly.
I don't see how it would be possible to post totally accurate standby waits. If someone enters the standby line and gets in the ride in 30 minutes, all that tells you is that the wait was 30 minutes 30 minutes ago. The current wait is going to depend on how many people have joined the standby line since then and how many people come with FPs.