There a lot of reasons possible. Is your topology fast enough to keep the drive writing? LTO 3 is 80MB/sec native assume 2:1 compression, you will have to feed it 160MB/sec. More realistic is about 1:4:1 which brings you to 114MB/sec, about a full Gigabit. Now of you deliver data too slow the drive will slow down and still keep in streaming but there might very well be 'empty spots' on your drive whenever data is not delivered for a second.
In addition about compression, assuming you do full disk-image backups, note that at least on Windows having for example a 1TB drive with 900GB free does not mean the raw image of the disk is mostly empty. If the disk had been filled up completely at one point in time, deleting files does not actually delete the data from disk. RAW imaging means all that 'non-existing' data is backed up as well. That might lead to unexpected compression results. Sure enough, if you make a RAW image backup of a 1TB disk which really has 900GB of zeros your compression will be awesome.