Others would be much better at breaking this down into the minutia then me but here goes my attempt.
By doing a weekly carb up you bring insulin into the game which has many positive effects over a strictly low carb diet. IGF has a very important role as well as testosterone in males which is lowered with continual low carb-ing.
Most specifically carbs or rather insulin plays a very important role in T4 to T3 conversion. Without the weekly burst of insulin to act as a "hormonal reset button" T4 would not convert to T3 and testosterone would be lowered.
I haven't read any research papers if you are looking for citations.
I basically just summed up the idea behind the Anabolic Diet.