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There definitely is a difference in manufacturing quality, and I would presume it's mainly due to residual gas, but maybe also cathode quality. When Admiral had a tube plant, they made some of the worst; Motorola was somewhat better for longevity, but still quite a ways below Rauland (Zenith) at that time.
In the case of the Saticon camera tubes in the 1980's tube-type HDTV cameras, the tube was rated for only 600 hours before they might begin to go bad - and it was recommended to run them for a few hours every 6 months or less - no long periods of unpowered storage. Again, I think this was to eliminate effects of residual gas. At the Zenith lab, we preemptively retubed our camera at a cost of $85,000 after about 800 hours. The old tubes were kept on the shelf as backups in case of a catastrophic failure, but of course they didn't get fired up after that, so may have gone bad anyway.
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