When a planet revolves around a star, it will have a fixed time period if the only gravitational force acting on it is by the star around which it orbits. For example, if in our solar system, only one planet were there, the it would have a fixed time period. But since there are more than one planet, the gravitational force between the planets affects its time period. In our solar system, this effect is in the order of seconds and hence negligible.
PH3C is 2300 lightyears from its star. and it has massive neighbouring planets such as PH3B and PH3D whose gravitational force influence is much higher. Since their relative position changes at different times, the mutual gravitational forces also changes. In simpler words, there are gravitational force acting on PH3C apart from the gravitational force of the star around which it orbits. So the time period varies.