It's been my experience that they calculate it all stupid, then react to that miscalculation in a dumb way, and inconvenience everyone as a result.

Suppose they have a product they buy for $8 and sell for $10. They get the number of all that got bought but not sold, which may include ones that weren't actually received or that were damaged during shipping and stocking or just left out and went bad or accidental "theft" or actual theft, and they multiply that number by $10. So now on paper they have "lost" a bunch of money to theft, even though they actually lost 20% less than the number they came up with AND they have no way to know how much of it was theft versus other shrinkage.

Now that they have this exorbitant number, they can use it to justify buying a million anti-theft devices. Which reduces the amount people buy because those things suck, but they can blame that on shoplifters too because it's all their fault that the anti-theft devices were "needed." And in the end none of it matters anyway because 1 theft is not the same as 1 lost sale to begin with.