This was explained to me before and my understanding is that Pekiti Tirsia broke off from the system of the Tortal family. They then altered the name but they are essentially the same thing.

Dekiti Tirsia Siradas is the original system, and if there's any difference Dekiti tend to focus on very large steps/movements like stepping off to the side and turning 90 degrees whereas the Pekiti takes on a more grounded stance.