It's really hard to explain, but when you have the wastgate flap openging horizontally, but the flow passing through it vertically, as it does on an RST, then you divide the wastegate, the flow will be restricted and you get boost creep. It's SO much easier to explain face-to-face!! LOL.

Basically, the wastegate only opens a small amount and it needs all the room it can get inside the turbine housing and by dividing the wastegate from the turbine housing, you restrict the room it has to flow.

Complicated eh?