What shrinkage actually measures
Shrinkage is the gap between the stock your records say you hold and the stock a physical count finds. It is not one problem: it is theft, damage, expiry and mis-keyed sales bundled into a single number, in roughly that order of difficulty to fix.
Expressing it as a share of sales makes it comparable month to month and between branches. Expressing it annually makes it impossible to ignore, which is usually the point at which a shop does something about it.