Not being a math type, I can’t explain why it happens, but it does happen with any antenna.
One of the specifications - front to back ratio - is a measure of the phenomenon you’ve observed.
Antenna design is a tradeoff between forward gain and F/B ratio. An antenna with a high forward
gain won’t have the highest F/B ratio possible. Conversely an antenna with a high F/B ratio won’t
have the max forward gain possible.
Exactly the sentiment of many when it comes to RF in general.
Here’s a link with info on antenna theory: http://www.antenna-theory.com/
The author says this about himself:
I am a practicing antenna engineer, with a PhD in antennas and I have worked for many years in defense, university and the consumer electronics field as an antenna engineer.
but he’s kept the material to an easily “digestible” level.