Iterating worklogs, Xporter exports all work logs (since start).
However, on those iterations it's possible to export billed hours (tempo timesheets plugin).
These billed hours are returned through a servet api and it only returns billed hours from current "open period" (since start of month or year, configurable in JIRA admin).
It should be possible to get all billed hours so it's consistent with the Xporter exportation.
This is configured with parameter "dateFrom" (doc)