How to maintain the same rpm at different altitude?

[tyyshxxx]

I am studying the FAA handbook and that under the topic fixed-pitch propeller, it says "

As altitude increases, the throttle must be opened further to indicate the same rpm as at a lower altitude"

Why is that? Isn't it when density decreases, lower power is needed to maintain the same rpm?

[AerocatS2A]

The lower density also results in less air available for combustion in the engine. This effect is larger than the reduced drag on the prop.