The difference is in the triggering device - it's a totally different mechanism. While I've never had a G70 apart, I did get a glimpse inside the G800, and I suspect that they use the same or similar type of triggering device. Big difference between the two machines in that respect. And, I'm really not sure which is more effective, electronically. While key velocity triggers the volume played, there are other factors such as after touch, and SA voice effect triggers that apply to Yamaha's key system. It's a fairly complex system, and I for one, don't have the answers for why any manufacturer uses the systems they use.
PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!