The graph shows gain (in the two orthogonal directions) as a function of
spacing. For spacing = 0.14 wavelength, the graph shows a bi-directional gain
of about 4 dB (and then a front-to-back ratio of 0 dB).

Unless I did something wrong, on EZ-NEC the same array with 0.14 WL spacing
shows a free-space zero-elevation gain of about 6.4 dB in one of the two
directions, with a front-to-back ratio of about 10.5 dB.

Re-thinking on the results, the gain difference is probably due to the fact that
the ARRL Antenna Book graph gives dBd, whilst EZ-NEC gives dBi.

But what about the great difference in front-to-back?

Tony I0JX