One possible culprit is creating a lot of documents in a very short amount of time. The date/time created is not typically stored on a document in a field. Instead it is identified via the UNID (see http://www-12.lotus.com/ldd/doc/domino_notes/9.0/api90ug.nsf/85255d56004d2bfd85255b1800631684/00d000c1005800c985255e0e00726863?OpenDocument). However, since UNIDs need to be unique, create enough documents in the future, and it will increment the UNID. If you're following where that goes, it means the created date will also get incremented, into the future.
If there is a $Created item on the document, this takes precedence (see http://www-12.lotus.com/ldd/doc/domino_notes/9.0/api90ref.nsf/f961929dc6dc14dc852561bf0067d1c4/85255d56004d3f63852564da001213e9?OpenDocument), which is what you would typically do if you set the UNID (instead of letting Notes/Domino automatically calculate it) and want to keep the created date/time.