I don't think they have many left of anything in LTS (which is why there are limited sizes and colors available) so if even a dozen people put, say, a size XL in black in their cart and don't check out, that's the whole inventory in that size/color tied up.
Lunchtime specials aren't regular inventory that QVC offers at a bargain price for a few hours, they're clearance items (most of which have been on "secret" limited quantity pricing for a few days or weeks prior).
And considering how many threads are started by people complaining that they got a letter asking them to please cancel less, apparently a lot of people do fully check out the items in their cart, only to cancel the order(s) hours later.