Most of the difference you see in the welds is due to lighting conditions.

The difference you see between the main welds and the 'weld' holding the BB halves together is that the BB isn't welded together, it is brazed together. So there is a visual seam but it is much more flush to the main structure than the main welds that are actual welds.

As an aside, it surprises me how people are turned off by visual perceptions to things that were decided on a primarily functional standpoint. It's the engineer in me, but I appreciate that they used different joining techniques in different spots due to, what I assume is, perceived structural benefits.