I'd have to look at the antenna in my own antenna modeling program,
which I don't have right now to be sure of this, but I'd guess it has
something to do with the antenna being an integer number of wavelengths
above ground. 100 feet is almost exactly 10 wavelengths at 98.1MHz.

Do you have an antenna program that lets you look at the elevation
pattern of the antenna? My guess is that at 10 wavelengths high,
there's more energy in higher angle lobes which I would expect that a
line-of-sight coverage prediction program doesn't report anything
about.

Dan
N3OX