the under rating of cars weight limits never ceases to amaze me, most cars can absolutely haul a half ton safely but few are rated for it, most half ton trucks can haul a couple tons safely, yet are only rated for half of one.
She was loading bags of gravel into a Toyota Corolla.
Sure, it'll drive, but will it brake?
If she should crash into something there's also a huge difference in having a heavy passenger wearing a seat belt and loading the back seats with bags of gravel.
They probably assume people just stuff their shit into the trunk so I guess the weight limit usually also includes a safeguard for weight distribution limits.
They also add a bit of a buffer for manufacturing tolerances along with allowing the rating to stay the same throughout the projected life of the vehicle.