I dunno if there's anything special about it, but my Cyrix 5x86-120 runs fine when set as 133 MHz (33 x 4), even in a crappy PC Chips "fake cache" VLB/ISA/PCI motherboard.
I think Cyrix never advertised the 133 MHz capability because production quantities of the "120 MHz" 5x86es were so low to begin with, they didn't want to splinter the few available chips between those that should be labelled and priced as "120 MHz" and those that would be "133 MHz" -- nor did they want any overlap, either in price or performance, with the low end of their burgeoning 6x86 series.
Cyrix also had bigger problems to deal with than just speed, in that the Cx5x86 was really a Pentium-class processor shoehorned into a 486 socket, and a lot of older motherboards and BIOSes had problems supporting it correctly. Cyrix compensated for this by intentionally setting some of the Cx5x86's most advanced features to be disabled by default, to help ensure that a less-than-ideal system would have a better chance of being stable. But with a fully supportive motherboard and BIOS, using a software utility to enable these features can achieve a significant performance boost.