Your miss informed... All servos work with a maximum pulse width swing of approximately 600us either side of center. This means a maximum 1200 steps are available with the proper TX end-point settings and servo/surface linkage ratio.

The TX plays a part in this as you suggest, but PPM offers finer resolution than PCM. Most current PCM TX's output 1024 steps to the RX with MAX ATV or end-points. The servo swings approximately 1.17 degrees per step 1200/1024=1.17.