Furnace Low speed,

My furnace is Rheem, 2-stage 80% upflow. (75000btu 1st stage, 53000btu 2nd stage). multiple speed (jumper adjustable) motor
In the furnace manual, it says:
"The blower motor is wired for blower speeds required for normal operation. The LOW speed, which is for the low fire operation should NEVER be increased.. The HEAT speed is factory wired for regular High fire speed. This maybe changed if required to maintain proper rise range."
So from the above looks like blower speed adjustment (to get correct heat rise) is only allowed for the HEAT (i.e. 2nd stage) speed .
I guess my question is why are we not supposed to increase the LOW speed?
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