What you're referring to, I believe, is the hydraulic properties. Yes, it can be measured in Watts, but has nothing to do with the heat dissipation: it's just a measure of the power used to produce that kind of flowrate at that kind of pressure drop.
What makes the curves different is this: each part of a waterblock will be made up of different size openings, which can be equated into their hydraulic equivalents, in round tubes.
Knowing that, you should be able to imagine that the curve would be dramatically different if, for example, you compared a Swiftech block (open area, center inlet) versus Radius (center inlet, with varying number of channels, and channel width).
In other words, any block can be represented as a bunch of tubes, of varying diameters. If you have a bunch of small tubes, then the pressure drop will increase dramatically, with the flow rate. IF the block is equivalent to one large tube (and it never is THAT simple), then the pressure drop would follow standard tubing specs.
Use Darcy to calculate those pressure drops.
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