I stumble across this today.
I placed a .psd image in Illustrator CS5 at 100% at 600ppi. My link information is telling me that it is at 600ppi.
But when I do a 90degree rotation, my link information is now telling me that I have a picture resolution of 859ppi x 420ppi.
Why is that? From what I know and learned, a resolution doesn't change except when we change the scaling of an image but not rotating it.
This made me wonder because when we receive files from customer with embed images, how can I know that this image wasn't rotated and that the link informations panel is giving me the right numbers?
Illy may get confused when you rotate, expecting things to be as they were (unrotated). It seems that your image has a width to height ratio of some 10/7 (landscape), so when it is suddenly about 7/10 (portrait), Illy thinks it has shrunk in width (corresponding to a higher resolution 10/7, from 600 to 859 (it should actually be 857) and stretched in height (corresponding to a lower resolution 7/10, from 600 to 420.