Some interesting carb dynamics going on here, I think.
when the air velocity in the carb increases the fuel that is drawn in by the venturi increases in a proportional fassion. However the increased velocity through the venturi drops the air preasure and consiquentialy air density.
If proprorionaly more fuel is added to a proporionaly increased air volume the mixture does not change, but because the air becomes less dense the fuel ratio actually increases. At least unless there are compensation mechanisums engeneered into the carb to limit the problem in the form of multipul jets (ie. L/H or more), maybe even the shape of the venturi, position of jets, or restrictions in the air filter other parts of the intake, ect.?
This said lower rpms = a leaner mixture, given that the engeneered compensation mechanisums are not perfect or over compensating at that point.