Let's say I try to take a picture of the following scene: a nice, blue river on the bottom and a solid, grey building in the middle. It is a sunny day and I am in a large distance, so both "objects" are in the same distance for me. When I focus on the river (80% of the picture contains the river now) — I hold the trigger so the picture remains focused on the river — and I move upwards on the building (still holding the trigger), the picture is generally lighter (or differently lighter) than when I do the same vice versa; that is, focus on the building first and then move the picture downwards on the river (then, the picture is darker).
What causes this issue and how can I intentionally control this behaiviour?
My camera is Olympus OMD EM10. I use autofocus.