Cecil Moore wrote:
plane of the radials.

Just ran that test. There was 0.02 dB difference at +45 and -45.

Run the test again more carefully. You are looking at something wrong.

Perhaps you didn't look at the entire azimuth plot at 45 degrees
elevation.

There is a large skew with a 1/4 wl vertical over two 1/4 wl radials,
and it gets worse at higher elevation numbers.