Also, if he was in Mountain View, it's likely that he owned a home and that contributed a bit.

The tech sector stock growth has been amazing/unprecedented over that time period. His first few years of stock grants have tripled or more in value.

I'm betting it's something like $2.6mm. $1.5mm in saved income, $0.8 in stock appreciation on savings, and $0.3 in housing/misc.