There may be any of several factors at play here:
- Emirates training and SOP's encouraging conservative energy levels
- Individual ability varies widely across 900 airbus pilots
- Higher aircraft may have a stronger tailwind component
- Higher aircraft will have a higher TAS for a given IAS
- Good energy management as previously stated
- Lack of faith in the awareness levels of ATC, as a result second guessing even the good ones
So it could be any or all of those, if we had a cohesive well planned and manned ATC system with constant descents and prescribed speeds, we would not need to spend as much time dealing with the human and environmental variabilities