Point is it does not really do anything really well and is a compromise at best. A resonate antenna will outperform it.
You may be able to invent circumstances where it would, but simply being resonant
doesn't imply anything for antenna performance or efficiency. If it were a resonant
half wave on 20m instead of being a bit short, it would probably work a bit WORSE
in many cases.
Here is an example: using a 31' vertical on 20m with 75' of RG-213 to a tuner in
the shack and no balun at the feedpoint, maximum radiation is -0.05dBi at 20 degrees.
The coax loss is 5.335dB, so taking that into account the radiated signal is -5.38dB.
If we lengthen the antenna to 33.48' is is now resonant. Radiated signal increases
to +0.19dB, an improvement of 0.24dB. But even though the antenna is resonant
the coax losses have increased to 5.837dB, for a radiated strength of -5.64dB.
Your signal DROPPED by about 0.2dB when you made the antenna resonant.
As you increase the length further towards 5/8 wave (which isn't a resonant length)
the antenna gain increases and the angle of maximum radiation lowers a bit. In this
situation the difference among 31', 33.5' (resonant), or 37' is a fraction of a dB, and
the resonant case does NOT give the best performance, regardless of whether or
not you include the coax losses. If you don't care about a couple tenths of a dB,
you can say that all three of those lengths will work about the same, but it certainly
is NOT the case that the resonant antenna outperforms the others.
What makes the biggest difference is the coax loss - around 5 to 6dB in this case (about
half that with a perfect 4 : 1 transformer.) The reason that the antenna is 31' instead
of a resonant length is to keep the voltage down somewhat so it can be matched with
a relay-switched autotuner, which is the most effective way to feed it on most bands.
In that case, lengths that are longer than resonance will give a bit more radiation than
So, yes, the 31' vertical is a compromise, but so is any other antenna, regardless of
size: you pick a set of properties that meet your needs, even if they are different than
those that others would pick. The behavior is relatively predictable, rather than being
contrary to the laws of physics.