When measuring capacitors at HF and UHF you get higher values of
capacitance than at audio frequencies. This is due to the inductance
and inductive reactance of the connecting leads between the capacitor
and the measuring instrument,

Furthermore, the capacitors themselves when connected into a practical
circuit, in series with their connecting wires, behave at HF and VHF
as if they have larger capacitances than their audio frequency values.

The actual values of capacitors do not change of course.

Large capacitors (1000 pF) are affected much more than small values
(10 pF).

The dielectric constants of low-loss plastic insulating materials are
constant between AF and VHF. Apparent changes in capacitance are NOT
due to changes in the dielectric constant.

On the other hand, what does it matter. When adjusting a tuner one's
eyes are focussed on the SWR meter and not on the capacitor values.
And the same applies to automatic tuners which don't have eyes.
----
Reg, G4FGQ