Have to look at it with PID tuning in mind:
https://www.google.com/url?sa=i&url=...AAAAAdAAAAABAY
The chance of overshoot error where turbine speed drop is minimal during DCT shift is small since you are not far from steady state (desired full boost). Large loss of turbine speed during *most* manual shifts requires significantly faster response time and thus higher chance for error over target during rise. It's much easier to control steady state error when closer to steady state.