Bug in the way photoshop calculates halftones
Photoshop does not seem to be computing halftone angles ( or perhaps the origin of rotation?) properly. Here is an example. I took a 70% gray square and bitmapped it with round dots at 0 degrees, 30 degrees and 60 degrees and put them back together as cyan, magenta and black. On the left is what photoshop does. The rosette pattern on the right is what I would expect.