Let's put some context on these numbers and explain the misunderstandings by proline and fw190.

ms refers to the number of milli seconds between each frame of data. The higher the number, the longer the time between frames. 21ms is a longer time to wait than 18ms. So 21ms is a slower refresh rate than 18ms.

There is a common myth that good old PPM ran at 50Hz or 20ms between frames. But that's only an approximation to make for easy explanations of how radio control worked. Different tx and different brands ran at slightly different rates and servos were designed to cope with a range of refresh rates around 20ms, but they were not fixed to work at 20ms and nothing else - if they were, they would work with very very few tx! For example, Multiplex and Graupner Tx in 12 channel mode were generating PPM pulses well in excess of 20ms, the data log in my Weatronic module shows the Mpx tx is sending a frame to the module every 25ms, and every servo I used with it on ppm, whether cheap analogue or expensive digital likeJR8511 all worked perfectly at 40Hz, 25ms. Some tx such as 4 channel didn't need to wait the whole 20ms for a frame and could repeat a bit faster and servos had to work with those as well, so they can cope with refresh a bit faster than 20ms. Not much faster, but a bit faster.
Hence 18 or 21ms is fine and all my servos attached to Wea work perfectly on either.