I have a document with Arial (regular) in it. When I open it in Frame, the font visually (and in PDF output) is Arial italic. When I look in designer/catalog/font, etc., there is nothing indicating the font should be italicized. When my coworker opens the file on her machine, it is the correct font.
It sounds like there may be an automatic subsitution set in your maker.ini file. Check the console window to see if there are any messages to indicate that the substituion was made.