This is a possibility...
Some Pentium® III processors have the same core speed in MHz, but operate on different system bus speed. For Pentium III processors operating at the same core speed, the product that operates on a 133 MHz system bus has been given a "B" designator. All Pentium III processors with same core speeds, and are manufactured using Intel's 0.18 micron manufacturing process have been assigned a "E" designator.
Example: A 533B MHz Pentium III processor would have a system bus speed of 133 MHz.