Why? To answer that we have to comeback to question asked in the subject.
What is the "load"?
The load as the exponentially damped/weighted moving average of the number of processes, including threads, using or waiting for CPU and, at least at Linux, in uninterruptible sleep state in last 1, 5 and 15 minutes (see Wikipedia). The last part means that all processes/threads waiting for a disk (or other I/O device) will increase the load, without increasing a CPU usage. It leads to situation when the load lower than number of core/processes is danger. Let imagine few processes trying to dump important information on disks. Especially if all interrupts have affinity to one processor only (see this post) or just data are store in many small files. On the other hand, machine with very high load might be very responsive. Plenty of processes waiting to write information onto a disk not using a lot of memory and CPU in the same time. Just look at this picture:
If you want to know even more details of how the load is actually calculated
read this impressive white paper.
Links:
http://en.wikipedia.org/wiki/Load_%28computing%29
http://www.teamquest.com/pdfs/whitepaper/ldavg1.pdf
http://larryn.blogspot.co.uk/2013/05/cpu-affinity-interrupts-and-old-kernel.html
