Try imagining a camshaft lobe like a wall. The duration is the thickness of the wall, and the lift is the height.
Now, say the wall is three feet thick. How tall is it?
Or, the wall is 30 feet high. How thick is it?
Does this help clarify the difference between the two terms? Although there are common relationships between lift and duration, nothing is set in stone (pun intended). As KC mentioned, other countries that deal with sixes are often more adventurous in six cylinder cams, than a V8 loving nation like the US.
Regards, Adam.