This seems obvious to me. Many people have bitcoin wallets with only a little invested. In my case, I have a few wallets just to play around with. I assume all of those wallets count as "accounts" in the above, which would really drive down the amount of money the average account controls. However few people actually have wallets with large amounts of money. I could easily move far more money into my wallets, but I choose not to.
On the other hand, when we're talking about non-bitcoin wealth the numerator is much larger (everything you and me have as assets could count as wealth, not just what we've invested in a specific thing) and the denominator would be much smaller (every one of us only counts as one person, while bitcoin wallets could be created on the fly)
In other words, the above is quite the apples to oranges comparison.