TR-7 tapes?
Those are rated at 20/40GB meaning 20 GB uncompressed, 40 Gb at 2:1 compression, but you will most likely never see more than 1.2-1.3:1 in the real world
So what your are seeing is about what one would expect for newly initialized tapes - 19.3GB is ~ 20GB, allowing for differing defintions of what a GB is