Do you have XCode installed? If so try this from the command line:
That should open Property List Editor.
Click the triangle next to "Root".
Look for "AppleICUTimeFormatStrings" and click the triangle next to it.
If you do not have "AppleICUTimeFormatStrings" or there is not a sub-element named "2", that is why AMP is not displaying your clock setting.
You can actually set the format there if you would like.
I have attached a grab of both the Property List Editor window showing how my office Mac has it's time set as well as the Customize window for the Time Formats in the International System Preference on the same system.
Hope this helps.