Based on what I am reading, what is being described is "heel spring".
Similar to toe spring, heel spring occurs when the toplift of the heel does not sit completely flush with the ground. Or whatever flat surface it is sitting on. With the posterior (back) part of the heel slightly lifted.
In other words, the "pivot" is actually the breast of the heel.
Heel spring is something very hard to avoid in industrial / factory made shoes because more often than not, factory made shoes use a pre-stacked heel.
As opposed to a bespoke maker, who builds the heel stack layer by layer, carefully balancing the "flat-ness" by skiving and shaping each split lift before he adds the next one. This gives the end result of a perfectly flush heel.
Without this gradual heel building process, it's quite difficult if near impossible to get a totally flush sitting heel. The degree of "heel spring" can vary from maker to maker using the same construction methods, within the same maker but between different pairs of shoes, or even between the left and right shoe of the same pair of shoes. It's really rather unpredictable.
One "RTW" maker though, whom I find circumvents this problem quite well, is Vass. I think they do not use pre-stacked heels. Which is how they attain the flush-ness of the heel stack.
But does heel spring constitute a defective pair of shoes? Not exactly. It is an inherent, nearly inevitable part of the industrial process of making shoes. That arises due to inherently imperfect processes being used. IOW, we expect it to occur. It is the same as saying that GYW is an inherently flawed technique when compared to handwelting. It's not "defective" per se. But it's not perfect.
