Does the file include non-ASCII characters that do not show in your editor?
If there is a UTF-8 character that the web page (ie google WMT) cannot display in your browser then it will show a ? instead. This can happen with some editors - eg smart-quotes and long dashes do not always show on web pages unless they are spefically UTF-8 enabled.
View your robots file within the same web browser you view WMT (exmple . com/robots.txt).
Some editors allow you to switch on "code" characters, or you could read the file in a binary editor.