Small addition to Brenden's answer, you can read more about macro signatures here.
And back to your question, why this could have happened. How did you install Kentico? If you have taken backup of the existing database and used a source code from different code repository you may have had different value in CMSHashStringSalt setting in web.config. Or you have changed it accidentally.
Hovewer, when you install new Kentico installation via Kentico Installation manager it should not be the case.