Try and think of it this way. Say you carry over your A's and B's for 3 days. And you also saved your syns and you had 45 syns to spend. On the 3rd day, you would be able to drink 900ml milk, 150g low fat cheese, 6 slices of wholemeal bread, and 2 potatoes (based on a red day) and that's without any free food or even thinking about spending your syns.
The message you would be sending your body is starve for 3 days then binge for 1. On the "lean" days, your body would think it was starving and on the "binge" days, your body would be spolit and want to hold onto all the fat/calories, in case it gets "starved" again.
This is roughly how my consultant explained it. Carrying over a few syns is fine, but not the HEX's.