The short answer is:
exif title becomes the Zenphoto description
exif caption becomes the Zenphoto title.
The long answer is to look at the EXIF specification (http://www.exif.org/Exif2-2). Zenphoto maps fields based on their definitions. In the case of exif title it is unfortunate, but the definition of exif title is "image description".
Note as well that there are also possibly IPTC data fields that might conflict. There is only one Zenphoto description field, but multiple fields that are essentially deseriptions, so maybe some other field is overriding your exif description.
The original fields are all present in the database and can be used if you devolve to the object model level. You can also disable Zenphoto from processing metadata fields if they are "getting in the way".