Starfleet enlisted insignia differed from today's system more than the officer insignia did.
More in which way? The officer insignia are almost completely
different from today's system. The TOS movie pins bear no real resemblance to anything (great imagination there!), the TOS sleeve stripes have different numbers of features (that is, stripes) than the "corresponding" USN insignia for any given rank (generally, it's USN minus one, but not with Captain!), and the TNG pips sort of resemble the modern stripes (even if they are completely unlike the matching modern collar insignia) but LtCmdr is arranged differently (with the "narrow stripe"/dark pip at the outer end rather than in the middle).
Saying that the first dark pip in a Starfleet Chief Petty Officer collar plate equals the rocker allows us to speculate that the plate (as per late DS9) is a specific rank symbol, while the single dark pip on a uniform collar (as per TNG and early DS9) generically means you're dealing with a person who has earned his rocker...
Since Starfleet is such an officer-heavy organization, perhaps everybody knows the very few CPOs by face and thus they don't need the sort of exact symbols for their training and competence that today are part and parcel of enlisted insignia? In today's situation, the few officers are the ones supposed to be known by face, and their insignia tell little about their specific training; in contrast, the enlisted are one-field specialists, essentially tools in a toolbox, and their faces mean nothing whereas their specialty is everything, and is prominently displayed on their sleeves. If Starfleet is the exact reverse of that, the generic "I've got a rocker! Ask me anything about machinery! Or about anything, really!" pip would make quite a bit of sense.
Further fun detail: O'Brien's plate clearly has room for three pips/stars, judging by the placement of the first two. There's no room for a fourth. This is perfect if the three regular CPO ranks are to be expressed by this plate, and two pips is the middle rank. That is, SCPO... We don't know if "MCPO of Starfleet" even exists.