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It's actually not that uncommon a power supply design. Every design needs multiple voltages. Typically the main B+ for the tuner, horizontal and audio output stages, Boosted B+ for the vertical output and a lower B+ for the IF.
You have a few choices for the lower B+, power resistor voltage divider, separate secondary power transformer winding, rectifier and filter or do what this design did. Use tubes as voltage dividers. I really don't think the fluctuation is any big deal. It's going to vary as the tubes draw more or less current in response to the incoming signal.
Tube circuits are designed with a wide tolerance. I wouldn't be concerning if readings differ +/- 20% from what the service info shows.
Those caps are ceramic and very reliable. I'd be surprised if they are bad. 3,900 is more than close enough. They are just RF bypass caps.
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