When looking at the ground for mapping purposes at fairly close range, the beam must be widened vertically as well as having its energy distribution controlled so that returns from longer range (the top of the beam) are of similar strength to those from shorter range (the bottom of the beam). The strength of the signal vertically within the beam depends upon the square of the cosecant of the angle of depression, so more energy is radiated in the upper part of the beam than the lower part.
The sidelobe mentioned is a side effect and coincidentally can be used to check if the system is working. It is not accurate enough to determine height with. The statement is probably to do with a question in the exams
