Why does the Pixel size of a jpeg change on a different computer?

I have a Mac OS X version 10.8.2 and Adobe Illustrator CS6.

When i save a file "for web" as a jpeg with a specific pixel size (320x50 for example) and then send it by email to someone, when they open it the dimensions somehow change.

Even though on my mac when i "get info" from the jpeg the dimensions are correct...

Please how can i stop this from happening?

Thanks!