Why is there distortion for the same two geoJSON features with different base maps?

Question

When displaying two features, a polygon a (see geoJSON) and its minimum bounding rectangle b (see geoJSON), I get the following result when viewing from geojson.io:

The minimum bounding rectangle around the polygon looks more like a parallelogram than a rectangle to me.

However, when I upload these same features to OpenLayers, the distortion is gone:

Is this a projection issue?

Answer 1

The GeoJON shown on GitHub is projected using a Spherical Mercator projection. This done automatically is so it can overlay with other web-mapped data (e.g. I can see it is in Colorado).

The second image is a Cartesian projection of raw coordinate data, so it is not projected. So the bounding box preserves it's right angles.