Art Unwin wrote:
"---what actually creates the narrowing of the forward lobes?"

Destructive interference of energy outside the main lobe lobe which
concurrently causes constructive interference within the
forward lobe as energy is neither created nor destroyed but redirected.

Highly conductive parasitic antenna elements can approach 100%
efficiency. They re-radiate, having no load other than a complete highly
conductive short sircuit, nearly all of the energy they intercept.

The re-radiated energy is carefully phased to create the desired
directional pattern. This can be "cut and try" or it can be calculate
and try.

Best regards, Richard Harrison, KB5WZI