The reason is that the gemini does not have intermediate rates. You have Guide (generally 0.5x sidereal), center (maybe 20x sidereal) and slew (800x) sidereal.?
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Slew is 3.342 degrees per second... if NINA tells it to slew 3 degrees per second... it tries the next slower speed... which is centering... if you ask it to do 4 degrees per second, it will go as fast as it "can" according to your settings... which will be slew speed.
Ryan Noonan