One characteristic of lithium-ion cells is a phenomenon called self-discharge. The concept of self-discharge is simple: Take a cell, charge it up, measure its open-circuit voltage (OCV), and let it stand with nothing connected to it. Come back to the cell some time later and you will find the cell has a lower OCV, indicating that the cell is at a lower state of charge (SoC). If the SoC is lower, that means the amount of stored energy (i.e., stored Amp*Hours) in the cell is lower.
The loss of stored energy wasn’t caused by an external load (such as a light bulb or motor), but rather by self-discharge, where the energy was lost as a tiny amount of heat to an internal load inside the cell. This self-discharge, which is sometimes called leakage, reduced the SoC and ultimately reduced OCV. Note that self-discharge is a continuous process, constantly draining the cell and reducing the SoC and OCV.