This answer 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.
