The processor speed was equal to the bus speed times a multiplier . At this stage in the game, the multiplier and bus speed were both set via jumpers on the motherboard. There was nothing to prevent you from setting the jumpers on your Pentium 100 to a 2.0 multiplier with a 66MHz bus and running it as a 133. If the processor's manufacture quality was high enough to handle the higher speed, you could pat yourself on the back and move on to the next activity. It's all in the process Widespread overclocking in techie circles brought CPU manufacturing quality into the limelight.
Processors are mass-produced in sheets or wafers.
Due to the nature of the manufacturing process, some processors are of inherently higher quality than others. Quality varies from wafer to wafer and within individual wafers. If the processor performs well at this speed, it is tried at 100MHz. This process continues until the processor fails to perform reliably at a speed. At that point it is rated at one of the lower speeds at which it performed successfully.