2.4mm has a 1mm "d" in a 2.4mm "D". 1.85mm has a 0.8mm "d" in a 1.85mm "D". Both are 50 ohm coax lines. But if you butt them to each other, you'll create a discontinuity at the transition step. The discontinuity can be tuned by offsetting one step from the other, but when mating a 1.85mm to a 2.4mm, this hasn't been done and the discontinuity remains. A properly designed adapter will tune out the step discontinuity.