Why do Unix-heads say "minus"?

A couple of weeks ago I attended a talk on Git by someone who seemed to be from a Windows background. I say "seemed to be" because he kept saying "dash" when referring to command-line options. I then recalled something that I found curious in my early days of learning Linux; that is, when referring to options, the resident Unix-heads always said "minus". That is: rm -rf /var/tmp/bogus/junk Would be said "arr em minus arr ef" as opposed to "arr em dash arr ef". Why is this?