gain of patch antenna...

hello...
for calculating the gain of the antenna patch (one only) , I thought it was between 8-9db
they made an estimate for the calculation , they say "
1/-the existence of the ground plane (and if we assume that all radiations occurs in one half of the hemisphere) give 3db.
2/-another 3db is add since there are 2 slots.
3/-The slots are typically taken to have a length equal to the impedance width
of the patch and a width equal to the substrate height. Such a slot typically has a gain of about 2 to 3 dB
so the gain is 8-9db...

questions:
in (1/-) why we have 3db , is that the slot isotropic?
in (3/-) I do not understand where does this 3db ,because we have already computed the two slots (in 1/- and in 2/-)

thank u all.