Another possible explanation of muscle memory concerns certain changes in your muscles that regular training may produce. Your muscles may adapt in two ways that could translate into faster gains during retraining. First, you may be able to increase the capillary bed surrounding muscle cells, creating a greater blood supply to the working muscle. If this happens, and many scientists believe it does, you would then be able to enhance the nutrient (glucose, branch-chain amino acids, etc.) availability to the muscle cell. Also, you might remove the waste products of repeated muscular work and energy production (lactic acid, heat hydrogen ions, etc.) at a faster rate. Since these waste products can limit performance, with the increased capillary bed, you would be in a position to train harder and longer.
Either or both of these situations would probably enable you to create a more effective muscular stimulus. This is the key in terms of muscle memory. These positive changes from an enhanced blood supply would be restored soon after a comeback since the capillary beds would quickly reopen. Thus you would have the advantage of a greater muscular stimulus from the start of retraining. This would lead to a greater adaptation - stronger and bigger muscles - and give the illusion of muscle memory.
Second, the enzymes that are involved in important bio-chemical reactions may be responsible for muscle memory. For example, we know that enzymes in reactions leading to the storage of glycogen (your energy source during anaerobic work) can be enhanced with training. It is plausible that enzymes involved in protein synthesis may increase in concentration and activity following repeated muscular stimuli and damage. It may actually be those enzymes that have a memory, quickly returning to their former increased concentrations and turning on these processes earlier. If this occurred, you'd be able to work out harder, possibly recover faster, and gained muscle mass more quickly than when you first trained.