Well, there you have the essential vacuum-demo stinkeroo. Pretty much any time you do that kind of thing, the second vacuum will always get something. The key to looking good in a vacuum demo is always to go last! If you did the test the other way around, you'd pull up some cruft, too. A Kirby is easy to demonstrate with because of the demo filter tool, though. (There are similar things for other vacuums.)
The most scientific way to test this is what Tacony does - have a standard carpet square, weigh it, add a known mass of dust, vacuum a fixed number of strokes using a robot, then weigh it afterward. They almost certainly have results recorded for the Riccar 1700, though I doubt they're public. Almost nobody has the resources to test like that, though, so the kind of tests Blackheart did here are a solid stand-in.