I've been speaking English since I was 4 so I'm pretty confident with my English skills. However, when I visited my former university, I talked to one of the professors and found out she is now the 'Assistant to the Dean'. I started a conversation and asked her if she's in fact the current Assistant Dean. She corrected me saying, "No, I'm the Assistant to the Dean." Her reply felt more like an admonishment. I think she finds being called 'Assistant Dean' as insulting.
What's the difference? Did I do something wrong? Thanks for the help in advance.