2:1 compression is a Marketing thing, not an Engineering thing. See http://seer.support.veritas.com/docs/199542.htm
I normally figure about 1.3:1 as a RuleOfThumb, but 46 GB on a 40/80 tape is only about 1.15:1, so you are on the low side, but what you get is what you get. There are no tweaks you can use to get better compression.