Images in PDF doc created by Word appear darker in the PDF than in the Word doc. Any known reason?

All the images are grayscale jpegs. The original images appear fine in Photoshop CS2, and in Word 2013, yet in the PDF created by Word (image compression is turned off), they appear slightly darker (checked in both Acrobat Reader XI and Nuance PDF Reader. OS is Win 7.

Can anyone shed some light on this behavior?