I assume it is because ZWO is using some open sourced library that for some reason does not implement the guiding at an autoguide rate (see MC_SET_AUTOGUIDE_RATE in the AUX document below), which hides in the NexStar motor commands.
I myself have not seen an official documentation of the NexStar motor commands. See the reversed engineered ad-hoc documentation of the motor commands here (the author calls it the "AUX" command set):
http://www.paquettefamily.ca/nexstar/NexStar_AUX_Commands_10.pdf
From what I can observe with my mount simulator, instead of using autoguiding functions in NexStar, ASIAIR first sets the slew rate to 1x sidereal when autoguiding, and then sends the slewing commands (MC_MOVE_POS and MC_MOVE_NEG) to generate the timed pulses to autoguide.
The rate parameter in the slew command MC_MOVE_* is actually an index to an array of 9 rates, with the index of 0 indicating a "stop slew." ASIAIR hijacks index 1 to use for all of its guiding and slewing.
(Big problem is that it uses that same index #1 to do the 16x, 64x, etc. fast slewing; so when ASIAIR "forgets" to set the slew rate to perform guiding, the mount would slew to never-never land. IMHO, they should have used index 1 only for guiding and index 2, 3, 4, etc for general slewing. That would help prevent accidents that can smash your OTA into the pier when they introduce a bug that "forgets" to set the guide rate.)
Note that the "AUX" document distinguishes GUIDE_RATE (Sidereal, Solar, Lunar; which ASIAIR does implement for the NexStar mounts) from AUTOGUIDE_RATE (percentage of sidereal rate; which don't appear to be used).
Chen
