Theoretically, you must know the views in three directions to determine the shape of an object. But after studying engineering graphics, you can imagine the graphics in the third direction only by the views in two directions.
Quite a few drawings use two views, because two views are enough to make the problem clear, so there is no need to draw a third view.
The third view in textbooks is determined by two views, and his method is scientific. The methods in the book all come from scientific practice, and there are no problems.
It's good that you have doubts, but you should also trust textbooks.
I suggest you read more books or materials on spatial perspective, which will be of great help to your understanding of the three views!