I have posted this before, there is a certainly a big difference in feel between a Dahon Helio 2007 (alu) vs a Speed 8 (steel) & Speed Pro (steel). The steel frame has much less vibration and is more comfy over rough surfaces.

The Helios was so uncomfortable that I sold it for a Speed Pro & added another steel bike, the Speed 8. So yes, it may be a case of design difference etc but for the case of Dahon bikes, their steel models ride much better in my opinion but are heavier.