Dropbox folders explained.

The package installs dropbox into /opt/dropbox/dropbox-lnx.<ARCH>-<VERSION>.
When a user starts dropbox for the first time, this location serves as the
application location (dist). This negates the need to download the entire
client once per user.
Dropbox client is self upgrading, and the next time a new version is released,
each dropbox user will get a ~/.dropbox-dist that includes the new client.
The next time that the user will start dropbox, it will start from the home
dist. From that moment and as long as the user home dist remains, the client
will start from it, not from /opt. Only users with no home dist folder (new
user or deleted dist folder) will start the client from /opt.