The reason the first one didn't make sense to you is that you're forgetting
that the point of maximum drag is at the speed you're running right before
you jump on plane. At 9 mph, that boat is pushing tons of water. At 25
mph, that boat is displacing much less water.

I'm not forgetting anything. Even though I gravitate toward semi and full
displacement hulls, I have been on plane in many dozens of boats and the
numbers don't make sense.

Let's apply some logic.

If wew agree for purposes of discussion
that the boat is not on plane at 9mph, (and it most likely would not be), and
that we have only 500 rpm to "spend" to get the boat from 9 mph to 25, observe
what has to happen:

We will need to spend at least 100-200 RPM (maybe more) to climb the bow wave
and maintain a minimal plane. For purposes of discussion, let's say when we
throttle up enough to climb the bow wave we achieve 14 mph. The unspent 300 RPM
are going to take this boat from 14 mph to 25? Almost doubling the speed? No,
they are not.

The other example, a 2500 RPM difference between 6 mph and 27 mph is more
believable than a 500 rpm difference between 9 mph and 25.