To put in perspective, without going into too much detail, tours are planned years in advance, often before a show makes it to Broadway. Most venues have already announced their 2018-2019 season, which means that the 2019-2020 season for every major market is already finalized, and touring presenter are already looking at 2020-2021 and beyond.
The standard practice of a year's padding allows producers to the viability of the show on Broadway, to see at what level/contract they will run the tour, and to ensure that touring houses do not have a show booked for the fall that just flopped in the spring.
This coming week is the Broadway League's Spring Road Conference, which brings in all of the road presenters to the city to both look at potential shows for future booking, and be wined and dined by producers for Tony votes. I will tell you, for certain, that several of the shows from this season have had tour dates for 2019-2020 confirmed with venues across the country for quite a long time already, before they opened on New York, and the official announcements we have been seeing are to increase visibility as the out of town Tony voters come in this week.
As a useful FYI, lots of shows have off-schedule, added performances this week to try and let the road voters see as many shows as possible within a short span of time. These are often good performances to find cheaper seats, and wider availability.