True enough - there was a steady increase until "Realm of Fear" where the two bright pips suddenly dropped to a single dark one, and then another possible increase during DS9 when the single dark pip was replaced by a plate with three chevrons and two pips. So, ups and downs, but "wildly" might be an exaggeration. "Out of the control of the writers" would be better...
FWIW, the first and only clear-cut TNG reference to O'Brien's rank, in "Family", nailed him as Chief Petty Officer of some grade or another when he was wearing two bright pips. That is, Sergey Rodzhenko used those pips to identify O'Brien as "another Chief Petty Officer", Sergey being one himself. (Perhaps O'Brien was wearing special enlisted pips, deceptively similar to Lieutenant's pips except for some detail that our TV sets don't reproduce?
The ambiguous TNG reference, from "Where Silence Has Lease", has no good angles showing O'Brien's pips. But he wore two bright ones in the previous episode already, so Riker considering him Lieutenant wouldn't be incorrect. OTOH, Riker is speaking mostly to Worf in that scene. And the specific phrase that includes the rank reference relates to the tactical choice of beam-in location - an issue of relevance both to O'Brien and Worf. And Riker is mostly looking at Worf when saying this. Make of it what you wish... I tend to use it as the perfect excuse to claim that O'Brien's rank was never inconsistent in dialogue!