100% this! There is no real reason to believe that a statistically significant greater number of people are in any given park on 12/10/2014 than were in the parks on 12/10/2013, or 12/10/2012. Remember that for all the hype and hoopla about Disney's increased profits from park revenues, even they concede that much of that increase comes from ticket price increases. If actual attendance is increasing, it is doing so by a low single digit percentage. So if there were 30,000 people in the MK a couple of years ago, perhaps there are 31,500 people now. That is an increase of 5%. That increase cannot account for what people are seeing, and what TP is predicting. The real difference is that FP+ has been a game changer in terms of crowd distribution, (which was ALWAYS its goal). We, as guests, haven't quite caught up to what that means for us, and neither has TP. All TP can do is predict the overall turnstile attendance--not in hard numbers, but in overall "feel". With over a decade of data under their belts, I'll bet that they are far more accurate than we want to admit. It's just that a "3" before and a "3" now feel very, very different to the seasoned guest.
My guess, (and it is only that) is that human nature causes us to magnify the wait times when they are longer than we expected, and dismiss the wait times when they are shorter than expected. For example:
Wait time at Pirates--used to be 10 minutes, now might be 30 minutes.
Wait time at Space Mountain--used to be 70 minutes, now is 50 minutes.
In the past, your total wait time for both rides was 80 minutes. Now, it is 80 minutes. But perhaps 30+50 feels "more crowded" than 10+70.