Probably because of the 486 DX33 and 486DX/2-66.
You see, PCI was developed back in the last days of the 486. Early PCI busses ran at either 25MHz or 33MHz. In fact, some cards refuse to run at the proper 33MHz, because they only had to be designed ot work with 25MHz busses back then. Even the BX chipset, which was designed for the PII with a max bus speed of 100MHz, had the 1/4 ratio for PCI in order to run such cards.
Now it's important to remember the 33MHz is actually 33.333...continuous properly, as a ratio of 1/3 of 100. So if computer geeks knew how to use rounding off rules, 66MHz would have been called 67MHz, because it's actually 66.6666...continous.
And that takes us back to the lowely 486DX-33 and 486DX/2-66, with their 33MHz system bus.
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