Al, WA4GKO wrote:
"Why does the reactance peak earlier than half-wavelength? Can it be
mathematically prtedicted / explained?"

At resonance, phase shifts abruptly from leading to lagging or vice
versa as resonance is crossed.

The resonant length depends on wave velocity along the antenna wire and
is a function of how thin the wire is. Resonant length is also shortened
by increased capacitive effect caused by the voltage pile up at the
open-circuit wire end. There are formulas involving wire length to
periphery ratio but these values have been already calculated and
plotted on convenient charts.

My 19th edition of the ARRL Antenna Book has Fig 8 on page 2-5 which
gives values from 94% to 98% of free-space wavelength for a range of
1/2-wavelength to conductor diameter ratios. Often the antenna is
shortened to 95% for "end effects" to avoid formulas and charts and in
most cases this is good enough.

Best regards, Richard Harrison, KB5WZI