make contour polygon from points

Hi, I tried to generate contour polygons from a point shapefile, here are the steps:

1. use 3DForcer to assign one attribute from the shapefile as Z value to make the point 3D;

2. use ContourGenerator (attribute LDEN has values in range (-20, 120), I have set the Interval to 5)to get contour LINES, i only use points as input, no breakline is provided;

3. Option 1: then use HullAccumulator -> GeometryCoercer to generate the boundary . After this, use Aggregtor to combine the contourlines and the boundary line, then the AreaBuilder to create the contour polygon, this options looks fine; Option 2: after step 2, directly use AreaBuilder to create the polygon, then I found the contour polygons are overlapped. For instance, contour 55 contains the contour polygon 60, 65 and higher.

Question 1: why is option 2 generate polygon like this? Why option 1 did the good job?

4. followed by option 1 in step 3, those polygons contains no values. So I use OffsetCurveGenerate ->CoordinateExtractor -> VertexCreator to generate points on both sides of each contour line to store the contour value. With overlay operation and statistics, each polygon get the min-lden and max-lden

My question 2 is: I noticed that the outmost polygon has a min-lden -20 and max-lden 55, looks like the inbetween polygons are all dissolved. Why is this happened?

Thanks for helping!

(I have uploaded the script and also the screenshot of the script)