There is a different font is some places (only on mobile devices, I'm using Chrome). It can be noticed in topic list, or the topic itself with usernames.
Yep, apparently on narrow screens the font changes... :-\
Going from memory, at the time the fonts used on desktop weren't available on phones. Android phones fell back to Roboto anyway, and iShiz fell back to Helvetica, IIRC. So putting that in the CSS made testing easier, since you could see WTF was going on.