Most tape drives these days advertise capacity with an expected compression ratio of 2:1. If you are backing up compressed files (.Z,.ZIP, .ARC, .JPEG, and so forth) then the drive's own compression scheme will not be as effective. For these cases, the actual capacity of the tape will be closer to the "uncompressed" capacity.
A table from firstname.lastname@example.org (Herbst OMR) shows:
stated capacity standard tape length # tracks 80MB QIC40 200ft (normal) 15 120MB QIC40 300ft (extended) 15 160MB (rarely) QIC80 200ft 28 250MB QIC80 300ft 28For all the recording density is 12500 ftpi; max tape speed is 90 ips.
A second reason is that some tapes assume you will be taking full advantage of their "streamers". Streaming collects a number of tape blocks and writes them all at once, preventing the need for backing up the tape after each block. This positions the blocks closer together on the tape. If your backup program is slow, some streamers won't be quite as effective. If you hear the tape drive motor backing up the tape alot on writes, this could be the case.