Let me first say, i like the way the shirt came out.
Thanks for posting the images of the separations, it showed me clearing what was happening.
That i'm seeing, is a "light" or a "medium" amount of black generation, but there is a huge amount of under color removal, so while not much black is being generated, where the black is solid, all of the other colors are being removed. I've attached the range of photoshop black generation options.
Personally i like the rich black, verses 1 color of black. That is a matter of personal taste. If anyones personal taste is to use the single hit of black, buy quickseps.
One thing i do question, is what happened to the red car. The red car is VERY red. The color red is composed of near equal amounts of magenta and yellow, but somewhere in your conversion process, the tints used in the magenta separation are clearly lighter than those of the yellow channel.
I've attached an image of the separations recombined to show the color shift which resulted.
fred