Others have already shared some good observations. Namely, if there's a lot of "free time", then it seems to show up in the touring plan in one big clump, rather than being evenly distributed throughout the day. If you add more attractions or "breaks" (for something like shopping, looking at details in a certain land, stopping for popcorn, playing in the grass, or anything that isn't considered an attraction, show, or meal), then it will fill in that time. If there aren't enough items on your touring plan to fill in all the hours (and you're staying for something later in the day like a meal and/or fireworks), then the optimzer doesn't know what to do with all the extra time.
Regarding 7DMT, there is a rush for it first thing in the morning and it's hard to predict exactly what your wait time will be. If you want to watch the morning Welcome Show before entering the park, then you'll be behind a lot of people who will queue up in the tunnel and start walking down Main Street as soon as they are allowed to (before that show ends). If the Welcome Show is less important to you than seeing the Mine Train with minimal wait, then position yourself in one of the tunnels under the train station and you can head there first. Of course, that also assumes that you and those in your party can walk relatively quickly. You don't have to sprint and be the first people to the attraction, but you also don't want to get passed by too many people.
Some more about 7DMT at rope drop. If you choose to watch the Welcome Show or don't find a dash up Main Street all that appealing, then the wait isn't too bad (typically around 30 minutes) after the first rush of people goes through the attraction. If, for example, you want to see a short ride like Peter Pan first, then you could return to the Mine Train after the people who went there first thing in the morning and you'll still wait in line, but it won't be unbearable. It may be worth doing so once, since there are some entertaining things to see/do while you're in the standby queue and that's just about the only time to see it without waiting in a really long line or darting past everything if you happen to be at the front of the rope drop race.
I hope that makes some sense and explains why it can be difficult to suggest when to visit 7DMT.