sp_helpsegment pages don't add up. How can I fix this?

I'm trying to use sp_helpsegment to determine how full the transaction log is. It seems one would expect 'used_pages' + 'free_pages' = 'total_pages'. However, this is not the case with my results. Am I misinterpreting the meaning of these fields, or is the data stale? If it is stale, is there some command I can run to refresh the data?