For comparison, some batteries are known to have a longer life when powering a low-power device (such as a watch) than when sitting unused new in the box. The technical explanation for this had something to due with higher internal resistance when electrically polarized by a tiny current flow (as I recall).
In the case of lithium batteries, it does not pay to buy a spare battery for a new device. By the time you need it, the spare will likely be dead too. From personal experience, it does not work to keep both batteries charged and keep swapping them, because that tends to confuse the portable device charging algorithm so that the power meter becomes inaccurate and the batteries may not get a full charge.