Difference between Processors and CPU core
Solution 1:
An E5620 CPU has 4 cores on the die. With Hyper-Threading turned on, that gives you 8 threads.
Perhaps your machine has 2 physical E5620 CPUs? If it's not powered on, try opening the case and counting them.
If it's powered up and you can log in, try this: cat /proc/cpuinfo | grep “physical id” | sort | uniq | wc -l
2 physical CPUs x 4 cores/CPU x 2 hyper-threads/core = 16 threads