Where are you seeing the minimum rpm? There's nothing in the manual linking rpm's with auto throttle that I can find. I pretty well use the auto throttle all the time except when using an attachment using the PTO function. The only thing I can see dealing with rpm min/max is the PTO rpm range on the tachometer.
If it's the same thing as the center of the tach on the RK55 (says n/min under an engine symbol) I think it's just a label for the gauge. Don't think it has anything to do with the auto throttle.