No idea, but here is one fact and one thing made up!

Since the glue joining the parts is vertical, there is no force put upon it im normal circumstances so it is not there to increase strength in the obvious manner.

Now, any piece of wood, being a natural product, has areas of strength and weakness. For example, it may have knots. If the thing was 2×2 that knot runs across the width of the whole thing and forms a point of weakness. By using two separate pieces where there is a knot on the left hand side there is unlikely to a knot on the right hand side. So the piece overall is stronger. And the glue helps distribute the stresses so that a significant area of each side helps to support the other.