Slot antenna with additional metal plane

Hello everyone, I'm designing dual-polarized planar slot antenna using cst. The shape of slot in ground plane is simple rectangular. Simulation of my system shows me the gain which has two maximums of about 4.2 dBi. To increase the gain i put metal plane under my system and the gain became 7.5 dBi. Could anyone help me with understanding this effect? Or maybe books and papers to get such information. The distance between two metal plane is chosen to be 3/4 lyambda, freq - 2.6 GHz. I wanna know - does slot reradiate reflected waves of it's something else.