I bet they also have to keep increasing/decreasing the ratio of FP people to standby people depending on how empty or crowded the FP lines get.
Three times now, I've personally experienced the FP line being so long they basically cutoff the standby line for a time.
A few weeks ago, we had a Tower of Terror FP+. When we arrived, the FP line was backed up all the way past the first Mickey scanners. This wasn't a backup to scan MagicBands at that first scanner, the line was solid all the way into the hotel. The standby line was much, much shorter (it was only out to the very first dry fountain on the porch area) with a 20 minute posted wait.
They made an announcement saying something to the effect of "Due to an overwhelming number of prior reservations, your wait time to check-in at the Hollywood Tower Hotel will be longer than originally anticipated. We apologize for the inconvenience".
From that point, the FP line was the only one that moved. It took us 10 minutes to get to a pre-show room in the FP line. During that time, I never saw the standby line move even a foot.
By the time we got off the ride, we looked and saw the standby line now showing 45 minutes instead of 20.
A couple of days later, the same thing happened at BTMRR. The FP return queue was solid people all the way down the hill and out past the first scanners. There was even a queue line of about 50 people snaked outside just to even get to the scanners. It probably took us 3 to 4 minutes just to scan our bands for the first time. Meanwhile, the standby line was all of the way up inside the building and the wait was posted as 30 minutes.
After we got past the first scanner, we were standing at the bottom of the hill in the FP line and I overheard several people walking up the hill in the standby line wondering if they were in the wrong line since the FP line was just a solid queue all the way out of the door. One girl even told her father to turn around and go back down because they must have accidentally gotten into the FP line somehow.
There were no announcements this time, but all of a sudden the FP line started moving quickly. I don't think they let any standby guests past the merge point until the FP line was cleared. We ended up only waiting about 10 minutes total. By the time we got off the ride, the roles were reversed and the standby line was stretched to the bottom of the hill while people getting into the FP line were walking right up into the building.
Then, just last week, the same thing happened at Soarin'. Standby was posted as 75 minutes when we got into the FP line. The FP line was backed up so far we stopped before even getting to the area where the giant screens are in the standby line. The end of the standby line was about even with the first screen at that time. Not long after getting in the FP line, they made an announcement apologizing for the fact that the estimated wait time was being increased to 100 minutes. After that, they didn't completely cutoff the standby line, but I would estimated they were letting in at least 50 FP guests for every 2 standby guests. We ended up waiting 15 minutes before getting to the second Mickey scanner, and about 20 minutes total.