To emphasize
@Robb point - the age of the material, the age of the sealing they used to package the material - and the type of materials used - requires some understanding of how different materials can migrate or diffuse into other elements giving you vastly different results while separated by the very insulator used to keep the plates apart and insulated from each other with a medium of material that exhibits a specific trait that gives the capacitor it's "capacitance" or characteristic.
It's also why they mention "tolerance" the part can drift, or even when made, can exhibit this character - so they use it, then later it its service life - it's now changed to this. Why?
Well, look up the different ways corrosion or mitigation, migration - diffusion and reduction - all processes of taking one material and applying natural events or subjective events (Heat moisture sunlight as examples) and watching the way it degrades or is reduced - to what end and by what degree - is part of the answer to the question you ask,
There's Galvanic, Electrolytic and Oxidization as just several means of the mechanisms of the reduction or degradation of performance in a part.
You can mix two materials typically inert - or presume as such, and later the impurities within the two substances will cause a recombination and form new materials - which if pressure, heat or some natural event (water CO2) can accelerate the process.
What is interesting to think about, is taking a typical piece of metal; a bolt, for example and when exposed to stress, vibration, salt, water, heat and air - it rusts of course, but why rust why not something else?
The process is reduction, and the material is simply reverting back to its original state. But realize too - the energy expenditure we had to produce; we applied heat to melt it, added ingredients to make it an alloy of other materials to refine it and make it harder - yet it still corrodes and rusts away - that process of reduction is the very same thing even synthetic materials are subjected to - and reduction affects them in various ways too.
So, in some thoughts, this is more of the process of conservation of energy - not created or destroyed, what we "made" using energy, is now in the process of reduction as it is being "attacked" by elements we used energy to remove - are now trying to get back into it to revert it back into a stable state.
So when you look at tuning circuits - yes, they do change with age, but not just from bad components, ("Bad referring to poor construction) but the materials used in the components change as they age - again the impurities or even the mixture that is stable, but degrades over time not yet defined when they made it - like ferric-chrome - Microorganisms living in Fe poor regions can produce a type of energy from the transport of Iron from one form (a chelation effort from rock) to be combined with another metal forming into another alloy. You don't always plan for the worst, but these events are discovered only when things degrade over time and you need to know why.
