Alumni vs. Alumnus (Alum)

Can someone tell me why every college sticker or license plate uses the word "alumni"? As I understand it alumni is plural, so it refers to the graduates of the university. Why then do we use this term to indicate that we graduated from university A? I have a sticker from the Alumni Association on my car. That seems to make sense "The Alumni Association" encompassing all the graduates of the school. My question I guess is what do the stickers and plates mean. Are they expressing "I am an alumni" or is it "I am a member of the alumni"? What do you think?