I'm with bjornhall here. Let's ignore CSU aircraft as that wasn't the question.
It's simple physics:
For a constant airframe configuration (not varying drag), airspeed and RPM are proportional to the power inpot into the prop. Feed it more torque, it will spin faster, and the plane will move faster. And vice versa. Direct causal relationship.
If you change the engine's power output (black lever OR red lever), the rpm will vary (very slightly). so lean to best econ, and the revs (and airspeed) will drop slightly as the power reduces. Unless 65% lean is with reference to a different max, you're now producing less power - let's say 63%
If you regain that power through moving the black lever to get 2400rpm again, you have to see the same airspeed. But the black lever will be further forward in the lean condition; it's still probably more economical as you're burning the fuel as efficiently as possible, and not flushing fuel out the exhaust.
Remember that for those simple aircraft without EGT etc., one of the recommended ways of leaning is to 'lean to peak rpm, then richen it a bit' (paraphrasing)