Irregularities in both trains' timetables cause this:
1. SCR: TN has a blistering run with high priority and very few stops, so it overtakes AP with more stops and a slower run.
2....
more... CR: AP speeds up exponentially. TN maintains a blistering run.
3. WCR: TN slows down suddenly. AP blistering run continues and it catches up.
4. NCR: TN gets hammered by AP, SBC Raj, and other trains.
5. NR: AP reaches NDLS hours before TN.