Then how come it doesn't give us 20db of forward gain?

It does, if you (incorrectly ) define forward gain as being relative to the
backside.

As I said, the signal (that) now no longer goes out the back gets its
power---redistributed--and that redistribution is like pushing the air forward.
It doesn't -- all-- get concentrated in one spot up front, thus the forward
gain isn't 20 dB.

73,
Chip N1IR