One limitation of the data is that it doesn't tell you the degree of the departure (i.e., 10% below the mitigated range vs. 80% below).

This deviation is pretty substantial, but it's important to realize that if the officer received a longer sentence that he would have had to go to a state facility instead of the Philadelphia jail. It's pretty common for judges to round down to 11.5-23 months to avoid sending someone upstate, which would be even more dangerous for a police officer.