Unit 902 is the Mirage 2000-5 version wich has the 2 DEFA 30 mm internal guns. The 2000C has them too but is not in the OOB. I suppose it's considered an air to air version and anyway you can't put all the version with all the possible loadouts for obvious reasons. The 2000D and N don't have any internal gun but could be equiped with gun pods.
Reference:
http://www.vectorsite.net/avmir2k.html