The answer is because the post-exercise anabolic window is not constant, but diminishes with time. Additionally, there are diminishing returns to calorie consumption beyond a certain point. This is because 1) you can only digest and absorb so many calories at once and 2) you can only gain so much muscle in a given time period before you start trading additional muscle for additional fat.

So, if you follow the standard lean gains restrict feast/ binge fast method, it makes sense to eat lower calories on rest days because if you eat the same surpluss on training days as you do off days, you'll likley incur some low quality gains (additional fat, as well as muscle).

Personally, Martin's lean gains calorie cycling is too meticulous for me because I abhor counting calories. So if you are like me and don't like swinging from one extreme (low fat feast days when training and moderate fat fast days when resting), then just eat normally all the time and just add a higher carb meal post workout.