simple man,
the reason why there is a low and high TDEE is simply because I implemented calorie/carb cycling into the calculator. This is done to curve adaptation and the starvation response if you decide to go for a VERY low tdee.
The idea is that you eat at LOW tdee for 5 days in a week (say, MON-TUE-WED-SAT) and eat at HIGH TDEE on THU-SUN.
High TDEE are also your higher carb days (and lower fat for that matter). You should think about training large muscle groups (like legs and back) on or the day after your HIGH TDEE days too.