The fact that you can choose "Georgia" in a list of fonts provided by the user interface, does not mean the reading software know what to do when a book specifies "Georgia" as font-family.
I know it sounds absurd, but unfortunately reading software and GUI are not that well coupled. It could be that the internal name of the font (the one that the reading application recognizes) is not "Georgia" but something else, like "Georgia (MS)" or "Georgia Regular"... and for convenience the GUI displays just "Georgia". Or it could be that the font is not even accessible for font-family properties can only be selected as a default font.
Why don't you just remove font-family from the CSS? That way the user-selected font should prevail.