If you were using the default [Smallest File Size] in both cases, that preset does handle grayscale and color images differently:
It also converts everything to sRGB, so in the end your grayscales get converted back to RGB color. It maybe the effective resolution of your grayscales is just under the 225 threshold?
Also, if you are using CS6 or later you can convert a layout to grayscale on export without converting the images to grayscale in PS first (unless you want to adjust the tonality of the conversions). Something like this in Output:
This seems like a good answer, unfortunately I'm trying to export this by doing a "Print Booklet", and doesn't give me the compression option. Also, am I supposed to just change the Bicubic Downsampling to the same as for color?
and doesn't give me the compression option
When you create a PDF via the print dialog the compression would be handled by the job options setting. On OSX they are set in Distiller, not sure where they are in Windows.
You can also Export form InDesign and use AcrobatPro's booklet printing
Same place, Rob. But as you already pointed out, Acrobat's booklet feature is a far better way to do this.