Full disclosure, I am pro immigration, against a border wall, think we should have a large scale amnesty for illegals, exc. That said, the reason for this statistic is very obvious, and most of you can probably guess what it is but won't say it out loud.
Beside the obvious and unspoken reason, there anre also other reasons.
1: Deportation of immigrants who commit crimes means a very low recidivism.
2: Criminal immigrants will usual target their own community, who are less likely to report the crime*.
*Murder rate is usual the best way to measure the real crime rate for a group, as it’s the least likely crime to be covered up.