I don't think its fair to call it "padding". They're on the same die anyways and share the same memory pool through the same connections, makes sense they all have the same speed. I imagine Intel/AMD CPUs with iGPUs also share memory speeds and are both limited to how many ram channels you have configured. Apple very much could achieve that kind of speed by having more ram channels. Have the ram working in quad-channel mode, and you double the 192 GBps to 384 GBps.