[Omp] OpenMP speedup is too good to be true?

[Open MP]

Dear all,

I tried the famous Jacobi code on a 4-way Itanium/Linux box using
Intel compiler 8.0. No any special compilation options were used
except -openmp.

The measured speedup compared to the sequential execution are 1,  2.3,
 4.4, and 6.8 for 1,2,3,and 4 threads respectively.

I expected the ideal speedup would be at most n when using n threads.
How could it be actually much larger than n? What are the reasons
behind these observations?

Thanks in advance.

An OpenMP fan