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Probably because the CRT needed "waking up". I've seen many CRT's, that have not seen regular use in years, that tested near dead upon initial testing. After letting the filaments "cook" at slightly higher filament voltage, the emission came back up.
There were two types of "instant on" in TV's from the '60's and '70's. The first type kept approx. half of the rated filament voltage on the CRT when the set was off. This method is usually found in tube sets. Many of the solid state sets kept the full 6.3 VAC on the filament at all times.
There are many opinions as to how damaging to the CRT instant on can be; but, I'm not a proponent of instant on and I usually disable the circuit whenever I can. The reason is because I've seen too many instant on sets with weak CRT's The main exception has been the early Zenith solid state sets of the early-mid '70's. I think those Zenith tubes would still function if a nuclear blast went off in the neck of the tube.
Concerning your tube, it would probably be a good idea to have it checked by someone with a good CRT tester/rejuvenator. If it's weak, there's a good chance it will respond well to rejuvenation, especially if the tester is a fairly modern one. It's been my experience that these delta gun CRT's usually last a reasonable length of time after rejuvenation with a good tester.
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