Difference in distance is not constant for point moving along a horizontal line parallel to the x-axis at a constant speed

Start with SpatialContext.calcDistance, SpatialContext.GEO, and the Haversine distance calculator. Reproduce the reported samples and compare the calculated deltas with the expected geodesic behavior. Done means determining whether the varying distance changes are correct and documenting or correcting the behavior with a verified result.

Hi,
I am trying to find the distance between a moving point p1 and a fixed point p2. Both of them are on the same y-coordinate. Point p1 just travels along the y = b line (n is constant). However, I noticed that while p1 moved at a constant speed, the distance value returned by the SpatialContext.calcDistance(p, p2) didn't change at a constant rate. Below is a sample of my data:

 p1               p2         distance

-141.854 , 52.94 | -106.75 , 52.94 | 1257.386
-140.194 , 52.94 | -106.75 , 52.94 | 1199.061
-138.534 , 52.94 | -106.75 , 52.94 | 1140.57
-136.874 , 52.94 | -106.75 , 52.94 | 1081.92
-135.214 , 52.94 | -106.75 , 52.94 | 1023.121
-133.554 , 52.94 | -106.75 , 52.94 | 964.1811
-131.894 , 52.94 | -106.75 , 52.94 | 905.109
-130.234 , 52.94 | -106.75 , 52.94 | 845.913
-128.574 , 52.94 | -106.75 , 52.94 | 786.6017
-126.914 , 52.94 | -106.75 , 52.94 | 727.1834

As you can see, the change in the x-coordinates between each consecutive snapshot is a constant value of 1.66 degrees. However, the change in distance (1257.386 to 727.1834) between the moving point p1 and the fixed point p2 (-106.75, 52.94) is not a constant value. In fact, the difference is increasing slightly from 58.3245 to 59.4183 between each snapshot. By the way, I used SpatialContext.GEO and Harversine distance calculator for this since my model is geodesic. However, since my point is moving in parallel to the x-axis, I expect the distance delta to be the same. Am I missing something here or am I wrong to assume that? Thanks.

By the way, I forgot to mention that the distance posted is the distance in nautical miles (which I converted from the distance in degree by multiplying the result with DistanceUtils.DEG_TO_KM * KM_TO_NAUTICAL_MILE).