Think it all depends on how many electrons they could put in the CRT in the first place. Some manufacturers (RCA, Zenith, etc.) had a propriatary method of putting electrons into the CRT which allowed for very dense packing, while others, like Curtis-Mathes, which didn't have much in the way of a research department, simply poured the electrons in with a ladle (or bucket in the case of larger CRTs). So the high-end guys with the sophisticated electron packing methods always had a much longer life expectancy for their CRTs. Of course, if any of them would spring a leak, the electrons were quickly gone and all bets were off. Hope this helps.
