I would think a lot of individual shareholders don't vote unless there's a high profile or high stakes issue like the Solarcity acquisition. Not sure if this data is available but would be interesting. This allows special interest groups to have an outsized influence on the vote.
The data comes from morningstar, and the adjusted data removed Musk's shares. To perform that calculation they just have total numbers of votes cast. (And I would guess that most shareholders vote by proxy, meaning individual shareholders are, in fact, voting.)