A quick answer is because there are 13 postures in the form:
1st segment: Grasp the Sparrow’s Tail, Seal it Closed
2nd segment: Drag Down, Brush Knee, Play the Pipa
3rd segment: Pluck or Drag Down, Twist the Joint, Elbow Strike, Shoulder Strike, Ward Off, Roll Back, Push, Press.
Another more fruitful approach is to consider the Eight Moves and Five Directions (8 plus 5 is 13).
The Eight Moves are as follow:
Ward Off, Roll Back, Push and Press (Peng, Lu, An , Ji in Chinese) are the core moves, and are associated with the four cardinal directions of the Bagua.
Pluck, Twist the Joint, Elbow Strike and Shoulder Strike are considered secondary moves, and are associated with the diagonals of the Bagua.
Notice that the third segment covers all of the Eight Moves.
The Five Directions are traditionally labeled Step Forward, Step Back, Glance Right, Glance Left, and Central Equilibrium. We can think of them as forward, backwards, right, left, and up and down ( ie the vertical dimension versus the horizontal dimension that is covered by the other four directions).
Each of the Eight Moves can be performed in Five Directions. So there are far more than 13 Postures possible, but the ancient Taoist were sticklers for numerology. The real result is Eight times Five, but the details are a little more complicated than that. As we’ll see, not all Moves can be done in all five directions for reason of bio-mechanics and simple common sense.
It is possible to further refine the Five Directions and split the direction of the body movement and the direction of the Move itself. For example, it is possible to step forward while performing a Ward Off upwards.
To repeat, the Eight Moves and Five Directions are crucial because from them come ALL the moves in the form.
For example, Brush knee can be deconstructed as a Roll Back backwards ( the English makes it seem awkward), followed by a Ward Off to the right and a Push forward.