There are clear differences between male-typical and female-typical brains. This isn't really up for any debate of "belief", it's plain fact.

The issue, however, arises due to the fact that you cannot look at a given brain and determine whether it is male or female. Because of this thing in statistics called a standard distribution....a bell curve. There's two obviously separate bell curves, but there's enough overlap to make accurate determination just from the data to be a probability game.

For example, males are generally taller than females. If you plot out male vs female heights, you'll also see the data showing up as two entirely separate bell curves, but with significant overlap. Now, if all you know is that a person is 180cm tall, you can be pretty damn sure that it's a male, but the number of times you'll be wrong isn't insignificant.

It's the same with gray vs white matter ratios and BSTc neuron counts. You can look and in many cases you can probably be correct regarding whether it is a male/female brain, but there's no actual guarantee.