I have a 13" MBP and a USB apple keyboard. The keys really feel different, despite same layout. External keyboard has easier to feel clicks and requires more pressure. Could be the use on my laptop, though (2 years vs 1 year).
You may be correct in that instance, then. The older USB apple keyboards likely use rubber membrane key switches rather than the scissor switch type. In which case, the feel of the keys would definitely be different. I personally was referring to the newer apple wireless keyboards.
I have a 2010 MBA, a 2010 Apple Bluetooth keyboard and a 2011 Apple USB keyboard (with number pad). They all feel different, even if the switching mechanism is the same - the Bluetooth one is best (by a long way).