What did I do wrong?

Thought I did good job caulking 6 months ago, but black spots are already showing up in the most wet area. The two photos show how most of the tub edge is still perfect except where most of the water runs down around the mid point.

I followed all the rules I know. Throughly cleaned and removed all the previous caulk, taped off the edges and used a quality 100% silicone tub and bath. What did I do wrong?