Whenever you make something out of solid wood, it will move. Drying wood will typically shrink, mostly along its growth rings, a little less across the growth rings, and very little along its length.
After a couple of dozens of lap joints, I have found that most of the elbows move in a particular fashion that one can plan for. This holds mostly for wood that is close to quartersawn; if you have freaky grain or flatsawn wood, your boomerang might move very differently.
The top picture shows the elbow of a right-handed boomerang in its original state, viewed from the top. When the wood starts shrinking, it will do so mostly across its width (orange arrows in the bottom picture). As there will be very little shrinking along its length, the cross-grain glue joint in the elbow means that the elbow will curl up in the two directions indicated by the blue arrows.
Given that the boomerang is right handed, this means that the angle of attack on both wings will increase, giving a little more lift. Usually, this just means the boomerang flies a little shorter, which is acceptable.
This is why I usually orient the joint in my boomerangs this way (the lead arm on top of the dingle). If you did it the other way around, it might yield a negative angle of attack that reduces lift, a condition that in my opinion is less tolerable than the opposite.
Of course, this is just a heuristic. At the end of the day, each piece of wood will move whichever way it wants to.