With Lantus, when the insulin is injected, it forms micro crystals that are deposited in the fat tissue. These mostly dissolve over the course of the cycle. However, a small amount remains and continues to dissolve into the next cycle, The insulin the same thing happens over successive cycles making the depot what amounts to a storage tank. When you increase the dose, it takes the depot a few cycles to catch up to the dose increase. The same is true in the opposite direction -- when you decrease the dose, there's still a bit too much insulin in the depot compared to the new, lower dose. As a result, the depot is "over filled."