The prospect object doesn't take into account _ notation for writing custom fields to Pardot, specified http://developer.pardot.com/kb/api-version-3/using-prospects#updating-fields-with-multiple-values
The prospect object doesn't take into account _ notation for writing custom fields to Pardot, specified http://developer.pardot.com/kb/api-version-3/using-prospects#updating-fields-with-multiple-values