The railway counter person must be lying to you. There is a facility to change the boarding station for tatkal tickets also. Your ANG quota for tatkaal is only 2 and is shared with other station enroute. Whereas if you book tatkal from NDLS to SBC in Sleeper , 288 tickets are available. But you need to go to the ticket counter early morning and see that you are 1st on the counter at 8.00am. However, if u wan to reach banglore on 23rd , the tatkal is already eaten up.