Radiation pattern of an inverted F antenna IFA

In Balanis' book, the working principle of a inverted F antenna is similar with slot antenna. But why does this the radiation pattern look like a Y-direction placed magnetic dipole?

Why isn't it a X-direction placed magnetic dipole?

What makes the nulls at +Y and -Y directions?

Why is it omnidirectional on X-Z plane?

I'm a bit baffled because the diagram doesn't show a ground.

Getting the antenna to resonate is different than getting the antenna to radiate. You can have a nice S11 and the radiation have '0-1%' efficiency. There must have been assumptions made.

Generally speaking, it's all current flow and field termination. The ground areas where the current flows the most and are furthest away from the monopole will most likely have the biggest lobes. Monopoles closest to each other will have small lobes/nulls.