Another reason pngcrush considers PNG files to not really be PNG files is if you accidentally export them with layers intact. If you inspect the file in Finder, look around the More Info area (Dimensions, Color Space, etc.) for an item called “Layers”
It might say something like:
Layer 68, Title Banner, Group 26
This happens when designers sometimes Save As PNG instead of exporting for web/mobile. And it completely destroys libpng (and thus pngcrush).
The fix is to properly export the PNG, or open the PNG yourself and re-export it as a “flat” PNG.