Why is not all the triangle and the rectangle shaded? and please explain the diagram

FIGURE 13.22 Second-Degree Price Discrimination The seller offers the rst block of consumption (0 to Q1) at a high price (P1), the second block (Q1 to Q2) at a lower price (P2), the third block (Q2 to Q3) at a still lower price (P3), and so on. This enables the monopolist to capture a substantial share of consumer surplus (the shaded area)