Designing a pattern for a darted cup bra is all about the curve of the dart. If you make it like a triangle then when you begin to sew it, you will be left with a pointy cup and be able to pinch the ends of the bra and your boobs won't fill it.
The dart shape needs to have some curve to it. If you were to put them side by side, then you'd see that the dart comes slowly together at the top rather than at a sharp angle, this ensures that the curve of the dart continues up into the cup rather than abruptly end causing the unwanted point.
If you were to lay the darts over one another you would see how the dart comes out then goes slightly higher than the one with straight lines.
To gain an overview of the whole picture here is the shape of the full darted cup shape pattern.