The saving on a package listing is arithmetic over its own parts: what each service costs on its own, multiplied by how many of it the package holds, added up, against what the package costs. Both numbers are on the page and every part is listed with its individual price, so you can check the subtraction yourself.
Where a mentor has also set a “was” price, the package ignores it and measures against the parts instead. A discount you can add up beats one you have to take on trust.
The part prices are captured when the mentor last saved the package, not read live, so the total on the listing is the total the package was actually built against and cannot move under you because of an unrelated discount elsewhere on that mentor's storefront. If the parts do not come to more than the package, no saving is claimed at all - and a package that costs more than its own contents is refused at publish rather than sold.