The 'juicebox.ttf' and 'juicebox.woff' font files are generated by the IcoMoon App and contain glyphs used as gallery icons (such as those on the Button Bar).
You can check out the contents of these font files using the online FontDrop service.

Juicebox does not use its own custom font for gallery text. The 'juicebox' font is used only for gallery icons.
Juicebox sets the font for gallery text in the 'theme.css' file (on lines 61 and 1179) to:

font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;

Roboto is very similar to Helvetica so maybe this helps to explain things.

For reference, more information on the use of custom icons can be found in the Using Custom Icons support section in the Theming Guide.

Please note that the line numbers above refer to the current version of Juicebox-Pro (v1.5.1).