The actual guidance is in U.S. Code, Title 10, Subtitle A, Part III, Chapter 105, Subchapter I, Paragraph 2123. In a nutshell it outlines four options (number four has never been used by any of the Services) with regard to the incurred obligation. Those options are, pay back the money, serve the time on active duty as something other than a medical officer, or serve twice the time in the Reserves. There is also some wiggle room for compassionate release in the case of <usually> medical issues so there is no recoupment.
In the Army, the HPSP office looks at each case on its own merit, and the desires of the member. The HPSP office at OTSG makes a recommendation to our Personnel Office, who makes a recommendation to the Sec Army (ASA M&RA), who makes an irreversible decision on each case. The OTSG recommendation can be (and has been) overturned at each step along the way. Which will account for variances along the way. HPSP will look at a medical drop from the position of where is the student in his training (M1?, M4?), and the diagnosis. Then make a recommendation from there. Other admin types in other offices seem to see things a little differently.
@Lets_Run: In the Army the Medical Service Corps is and has been over strength, meaning that unless you were previously in the MSC, chances are that it will not be offered up as an AOC at this point in time.
