IF:
1. Unreserved sleeper classes are issued to defence personnel only OR
2. Issued selectively to SPECIAL CATEGORY persons OR
3. Issued by selected station booking offices OR
4. Issued for selected trains
Then...
more... there is no logic or reasoning in this and one need not be advised to try. The simple rule is: buy a general tkt, approach TTE to allot a berth by paying the fare difference and resv charges. If there is no vacancy, the TTE will collect charges and convert the tkt to sleeper class and you have to fend for yourself during the journey. In some trains small sections are open to general public who can board the earmarked coaches with general ticket. The checking staff will collect the difference of fare only. This is due to quota allotment at major stations enroute.
However, in the entire dscussion there is no surety of getting a direct sleeper class tkt from booking office.