Sorry I didn't read this post earlier.
Yes, there is a reason for this phenomenon and it is not a theory at all. It is a scientific fact called gravity intensity variation (giv). It basically means that as you change your latitude (horizontal lines on a globe) north or south, the strength of gravity changes slightly due to many factors but mostly the centrifugal force of the earth's rotation. The closer to the poles you get, the more gravity intensifies. Scientists have documented this fact extensively over the centuries with the Early Greeks being the first to discover it. They observed free swinging pendulums showing a difference in how quickly plumb knocked over blocks lined up in a circle around the pendulum as they moved the experiment from one location to another. Gravity seemed to change on the plumb the farther north they got.
It gets pretty complicated because there are actually many factors that play into it, but if you are up to reading the exact scientific mumbo jumbo, go to
or get the book called GRAVITY ANOMALIES by Stommel.
The best thing to do to overcome this cosmic problem that plagues us shooters is just as the earlier poster said. Just rezero your gun anytime you go north or south of home!
PS. I think this phenomenon is also to blame for people all over the globe complaining that ballisitic programs and drop charts and cosine indicators aren't working like so and so says there supposed to! Also, it shows how Drag 1 computations might differ from what you actually see in the field. One thing I would love to get a hold of is a program for the Iceland military (if there is one) and see how far off their drag function forms are from the ones we use that were developed in the 1930's somewhere in New England. Being that far north, there should be a significant difference in drop and drag due to gravity intensity.