Our Crowd Calendar is based on wait times in the park between 11:00 a.m. and 5:00 p.m. and not on availability of park reservations. Disney controls how many park reservations are available, and it can change that at any time for any reason. We take park reservation availability into account in our wait time forecasts, but that is only one of the many factors that impacts wait times (and thus our Crowd Levels).
Please also do not assume that medium or low numbers on our Crowd Level scale imply that parks will be uncrowded. Disney theme parks are "crowded" by any conventional definition nearly every day of the year, and you will absolutely feel crowds when watching parades or fireworks, going to the most popular attractions, traveling through certain sections of the park, etc. Even on an average day, you will see attractions with 100+ minute wait times and areas with thousands of people. In addition, individual experiences vary widely depending on what you're doing, where you are, and your expectations: we have had guests in the same park on the same day report completely opposite subjective experiences in terms of crowds.
Although a visiting the same park on a date with a lower Crowd Level results in lower mid-day wait times, on average, there may not be substantial differences for the times of day and attractions that you plan to experience. There are better and worse days to visit theme parks, but how you tour (arrive early and use a Touring Plan!) often matters a lot more than which park you choose to visit.