Why can’t I change the “From” field in an ics file that I want invite others to?

Someone else prepared an ics file (probably on Windows Outlook) and asked me to invite others. When I click the invite button, I get a new window with the "From" field already filled in, but not with my email address (and not even that of the sender). The field is a popup menu with only one choice, the already filled-in one. Interestingly, if I send the ics file to someone else with Mac Outlook 2011 and that person tries to invite someone, the new window appears without any "From" field at all. Why is that?