After the four- and five-way interactions, participants said things like, "I kept losing information," and "I just lost track."
This compares interestingly with this article. The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information
The advantage of GTD is to dump everything but what you are currently working on onto paper/computer and out of your head.
Trying to keep everything in your head is not effective, once you have filled on of those precious slots with boss, spouse, kids, big project, little project, shopping list, and honey-do project. You are at the limits and just one more thing can push you over the edge.
Your humming along and then an accident or health crisis comes along and smashes your fragile system to pieces.
A dishonest management technique that I have seen used is to overload an employee with more things then anyone can possibly handle, and then fire him for "not performing."
Back to the original article, 4 variables are not all that many but like a computer tracking them takes more resources then just storing those 4 variables. sure storing 7 variables is easy enough for most people but tracking variables is much more then that. You are comparing them to each other, searching for patterns. Yes, you reject a bunch that don't work but you store ones that do or at least appear to.
When I have a design in my mind I keep a whole lot of things in my mind all at the same time. One reason I hate interruptions is that it all comes crashing down as the straw that breaks the camels break gently lands on the stack of stuff in my head.
I am going to keep this in mind as I management my projects to keep the number of variables small at any one time.