Hi Jesse,
The official data come from VIP. It is data that State and counties export out of their databases for the purpose of sharing with Google and other interested parties for free. The "unofficial" data, on the other hand, come from a variety of sources. They may be constructed from a voter file with hand-sourced access to polling locations, early voting sites and etc. For unofficial data, we may also need a lot of heavy lifting (such as join the data from various sources) to make it available.
Hope this helps!
Cong