This also occurred to me, but are enough really deported to account for an 18-point difference between total immigrant population (some of which includes US citizens) and immigrant prison population?

Plus, the study takes this into account. From the Mercury article:

"Kristin Butcher, one of the report's co-authors, said the low rate of incarceration could be linked to U.S. immigration policies, which call for carefully weeding through visa applicants and deporting illegal immigrants accused of serious crimes such as gang involvement and murder."

I don't know where this myth about immigrant crime comes from, because when you think about it, most immigrants come to the US to enter the economy, which is hard to do when you're behind bars. Most immigrants probably go out of their way to avoid trouble, not cause it. And even if the discrepancy found in the study is due to deportation, at least it shows that our immigration policies are protecting US citizens effectively; at least we know that the immigrants who don't get caught aren't committing serious crimes.