When used to center-feed a dipole, making the ground-side leg longer
than the hot-side leg increases the capacitance to ground on that side
and may tend to mimic the environment for which the tuners were really
designed.


It is probably because those tuners do not do well with a resonant
antenna. With one legg longer than the other there is more chance
that the antenna will be reactive and make the tuner happier.

73
Gary K4FMX