The number of clipping polygons shown in the Metrics view represents the total number of convex polygons created by your clipping attachments. When a clipping attachment has concave vertices, it is decomposed into multiple convex polygons.
In your case, you have two clipping attachments. The right one creates one convex polygon, while the left one, which you assumed would create three extra polygons, actually results in five convex polygons. This means that the left clipping attachment's vertices are decomposed into five convex polygons due to their concave nature.
For optimal performance, it is best to ensure that each clipping attachment is convex, resulting in fewer clipping polygons. You can find more information on clipping performance here.
