I have noticed that usually the RCMD changes to a higher altitude prior to the actual green S/C point on the ND. Does anyone know why.
RCMD looks forward up to a maximum of 500nm (250nm for 1000ft steps, 333nm for RVSM/ICAO/2000ft steps, 500nm for 4000ft steps) and recommends a level ONLY for that segment. STEP TO or S/C looks at the economy of the entire flight and will only suggest a step once it's 20% more efficient.
It's all in the manuals....
Highly recommend you follow the STEP TO unless ATC requests otherwise, then utilise RCMD as a preferred level, or MAX if you're forced up there (provided your speed won't change...maybe check LRC's MAX first).