That's pretty fascinating. I just assumed that the majority of their net wealth was in the form of MSFT shares. I'm now very curious what the other 2/3s of their net worth is held in.
It's very common for executives to sell shares on a regular (non-secret) schedule to avoid claims of insider trading or false news about "Bill Gates selling stock; MSFT must be in trouble!"
You can see them steadily selling off shares here: http://www.sec.gov/cgi-bin/own-disp?action=getissuer&CIK...