Having experienced it firsthand, I wonder if those numbers don't tell the full story. There are whole families basically going in and out of jail running the drug trade. I never got the sense that violent criminals had quite the "revolving door" effect.

Edit: then again, it's likely that drug offenders would be much more open about what they're doing time for. Now that I think about it there could definitely be positive selection bias (or whatever) affecting my perception.