Codon Usage

[Emma Macfarlane]

This is probably a very basic question. A colleague has isolated a gene
from a cDNA library, the sequence for which has already been published.
He has found the codon usage in his gene to be different to that in the 
published sequence although the libraries the two genes were isolated 
from are the same (human spleen cDNA). Is this a usual occurance?
Thanks in advance for your time
Emma