isobands generating countered some wrong results

[wxyer]

Please provide the following when reporting an issue:

• [1 ] The version of Turf you are using, and any other relevant versions.
  "@turf/turf": "^6.5.0"

• [ 2] GeoJSON data as a gist file or geojson.io (filename extension must be .).
  pdop.json (sorry,geojson extension file con't be uploaded!)

• [ 3] Snippet of source code or for complex examples use jsfiddle.
  const contourPon = isobands( featureCollection(pdopData), [0,1,2,3,4,5,6,7,8,1000], { zProperty: 'pdop', }, );

• [ 4] Verify this issue hasn't already been reported, or resolved in the latest alpha pre-release.
  the resulte give the three isobands,but the second isonband is which first and third merged,not difference with boundingbox,