Not getting all data on DAT tape

I have a W2K server SP4 with a Seagate DDS3 20/40GB tape drive using Veritas Backup Exec 8.6. I thought I am supposed to get at least 20GB of uncompressed on to 1 tape, but I am only getting 15-17GB of data with compression enabled to 1 tape before it pronpts me for another 1. On some tapes I may only get 10GB. Any help would be greatly appreciated. Thnaks.