maybe you can't read?
same TYRE: 18" to 20" WHEEL = harsher ride. fact
if you start changing WHEELS as well as TYRES at the same time, you might end up with tyres that, despite a sidewall being smaller (if you want to keep within oem specs for the electronic aids to work correctly) it could well be softer than the OEM tyres which incidentally are the RE040, one tyre with a very very strong sidewall (and now on 07 Zeds, the RE050 iirc, of which I do not know how the ride feels on std setup)
PS I find tyres with stronger sidewalls much better for driving, too.