Obama graduated from Harvard in 1991. Maybe he had declared a college deferment or something and it expired??
... or maybe Obama had his name changed or adoption reversed at this time?? Might be an interesting UIPA request to the HI DOH to see if he filed for an amendment to his birth records in June 1991.
If my understanding of deferment is correct, you still registered for the draft, but applied for deferment from being actually drafted. Regardless, a deferment would only apply in times of war and there was no draft used from 1980 to 1991.
There is no deferment for filing. Only for being drafted, if there was a draft. Students would be allowed to finish the current sememster, seniors could finish the academic year. But you have to register, unless you are on active duty in the military, in the hospital, in a mental institution, or in prison. Even then, you must register when you get out, if you are not yet 26. Even if you are disabled, or would claim Conscientious Objector status if drafted, you still have to register. Even if you joined the military at 18, served 6 years, and then separated, you'd have to register, if you had not already done so. (However they won't punish you by taking away stuff like student loans if you served in the military).
Besides, he would have been 30 in 1991. Registration is required for 18 to 25 year olds.