Alum siding - leaking through horizontal seam on sides

I have a 1978 Rockwood. Overhead cab damage and replaced as needed.

I wish to button this project up but just before I put the side paneling on - I noticed water dripping from the horizontal seam at tip of represented arrow. See photo. My thinking is that the seam should have water overflowing from the top of the seam if leaking from above (as viewed from the inside) but is it coming from within the seam.

How is this happening?

I must not understand how the seam is constructed...