I think it's about how and where you put on weight, and the order in which you lose it.
An hourglass will gain weight equally on top and on bottom, while the waist stays proportionately narrow (i.e., regardless of how many overall inches are gained, there's still a 0.7 or less hip-to-waist ratio).
A pair will gain weight more on bottom than top, while the top stays relatively slim and the waist stays proportionally narrow (0.7 or less ratio).
Whereas an apple will ALWAYS have a 0.8 or greater hip-to-waist ratio no matter how little they weigh. When weighing at a low/healthy BMI, the apple may look like a rectangle. Or, an inverted triangle, if the shoulders are broad and the chest just a bit wider than hips. Rectangles and inverted triangles are at the most risk of becoming apples with weight gain.
Even when I weighed 97 lbs, I still had a 0.8 ratio and finding jeans was difficult, though nobody would have said I had a belly. If the waist fit, the hips rear and legs were too baggy. I view inverted triangle as my primary shape because when I gain, I gain first in the upper body -- chest, arms, shoulders, back, chin, as the belly expands too, and very lastly my lower body (it always looks proportionately slim, no matter my weight or cup size). I say I'm primarily an inverted triangle also because I routinely take a size larger on top than on bottom, whereas a rectangle or hourglass would take the same size.
Also, in replying to the other thread, it occurred to me that some of the apple tricks I employ to hide my belly will just not translate to Zaeobi's shape, which to my eye is more hourglass.