WD caviar black 650gb speed low?

hi so my 2 week ld Wd caviar black 640 gb Hard disk is running at a spees of 91.5mb/s and Western digial says it runs at 113mb\s whats wrong ?  

I suspect that your drive may have 3 platters, whereas the faster version would have 2. What is the DCM on the label?

BTW, is 91.5 MB/s the maximum read transfer rate?