Different dimensions in Illustrator than Photoshop

Hi

I ran into a very annoying issue:

I have a PS file that is 1366X768 in 72 dpi.

I placed it into a document in AI and than saved it as PDF with no downsampling at all, because I wanted to preserve the best quality of it.

Now, when I view the PSD in PS, at 100% it is in a certain size and quality, but when I view it in 100% in Acrobat, it's much bigger and much worse quality. Of course that if I view it in less than 100% in Acrobet it's even worse.

So, why is there a difference between AI and PS (and PDF) in the on-screen sizes of images (at 100%) and is there a way to make them all the same?

Thanks!