Here's a guy who is leaning the bottom of the bag towards the camera, and it's more of a lean than you can claim for Montgomery. You know he's leaning it because if he were holding it straight, that is, perpendicular to the ground, you wouldn't see the bottom panel of the bag. But, you do see the bottom panel of the bag, so he must be leaning it. And it is causing the lower part of the bag to be wider than the top part. However, there is no visual distortion. There is no optical illusion. Our minds process the difference; we understand the discrepancy, the reason for it; and we take it for what it is, a rectangular bag. We don't get the slightest impression that the bag is pyramidal. We know it is rectangular.
How come we are able to process it without getting the wrong idea? Without being fooled? It's because the difference is so slight.
But, in the case below, the difference is larger- a lot larger. We are talking about at least a 50% greater width on top than at the bottom, and that is much greater than a lean could produce.
That bag looks pyramidal because it was pyramidal.