[8-Ball]

Let me try and explain it this way.

There are two ways to use a heat exchanger.

1. Dissipate a constant power.
2. Cool a liquid down to a constant temperature.

For 1, if we increase the flow rate, then the energy dissipated per second does not need to change, so the energy dissipated per unit volume of liquid decreases.

For 2, then we need to dissipate a constant amount of energy per unit volume per second, so if we increase the flow, rate, then more energy must be dissipated by the heat exchanger.

Just different uses of a heat exchanger.

8-ball