Maybe they just assumed you would go because you had heard about it around the office?
At my work you find out things through Email. Since I don't have Email so that makes it a bit hard so I have to find out through posters on the wall, asking and word of mouth.
In my department it seems like you are automatically invited to something because you work there. Still you have to double check if you wanna bring a date or a friend along.
Does your workplace have a social club? I recently just joined mine. Things are gonna be a little cheaper for me now when I go out to social club outings.

. For a non member things cost a little bit extra.