[This answer][1] on a question with a similar theme may provide an explanation for your observation:

> ..since your data is in Decimal Degrees, you will not be able to perform
> a linear calculation for the true area of your raster cells. Since the
> lines of Longitude converge at the Poles, the distance represented by
> 1 degree of Longitude decreases as you move north or south from the
> equator. You need to be using a projected coordinate system that
> preserves the area of your data. This will ensure that your area
> calculation based on the raster will be accurate to the ground area.

My first thought when I read your question was that using decimal degree measures to compare linear and areal values would not be sound, but admittedly, off the top of my head, I couldn't explain *why*. However @GetSpatial's answer makes sense of this.

  [1]: https://gis.stackexchange.com/a/59081/4968