framer,
While I don't know how and why Boeing scheduled those speeds, I would expect the s.e. driftdown speed to be the same as the s.e. climb speed, optimized for best s.e. gradient of climb/descent, without regard to fuel consumption, i.e. close to VMD. Why the holding speed is lower I don't know, but it could be due to the different engine configuration, i.e. both engines operating for level flight vs. one engine out, remaining engine at max. continuous thrust (e.g. different trim drag, different specific fuel consumption).
P.S. I was hoping to find some answers in Boeing's Jet Transport Performance Methods, but unfortunately there is nothing on holding and driftdown speeds, and chapters "Performance and FMCS" and "Speed Stability" are still "to be supplied". Perhaps you should ask Boeing?
P.S.2
Oops, holding is discussed in the "Cruise" chapter. There it says:
In the holding pattern, however, while the emphasis is still on fuel efficiency in this case we want to minimize the rate of fuel flow. Minimizing the fuel flow rate means minimizing the amount of fuel consumed while holding.
“All right,” you say, “we’ll just calculate the fuel flow at the speed for minimum drag. Minimum drag means minimum fuel flow, right?”
Sorry, no. It’s a little more complicated than that. Actually, the minimum flow will occur at a speed slightly slower than the speed for minimum drag. Let’s see why that’s true.
The explanation that follows is that the engine TSFC (Thrust-Specific Fuel Consumption) increases with airspeed.