On the VNAV CRZ page the OPT, MAX and REC CRZ data is dependent on the data that you have loaded into the FMC, namely Cost Index and the Winds/Temp data for the various altitudes/waypoints. You can see the Wind/Temp Alt Data by going to the LEGS page then RTE DATA and selecting a waypoint. It should then open up into a separate page with the predicted Winds and Temperature for up to 4 Altitudes. associated with that waypoint. The NEXT/PREV will scroll forward and back through the listed waypoints.
The actual calculation on the VNAV CRZ page is made using the data supplied for the current altitude with a slight bias of the current Wind and Temp to give a data set that is used for the calculation.
Note that the calculation uses the CI data for your particular airline and as such the same number ie CI = 100 may generate a different speed between airline A and Airline B.
When you put in an RTA you are overwriting the performance parameters by telling the aircraft to fly at this altitude to achieve a Way Point Crossing time of xxxx. Note that it comes up with a Max Speed that it will do and to achieve RTA the big controllable item to achieve this is speed.
Speed increase or decrease affects the altitude capability at a set weight thus the calculations on the VNAV CRZ Page are removed until RTA is either removed manually or by crossing the Waypoint