Two MacBook Pros display my webpage differently
Try clearing the cache for Safari (Safari-> Empty Cache).

The next thing to make sure of is that you don't have any extensions or custom stylesheets.

Also, depending on your CSS, the size of the screen and window could affect positioning. Make sure you're using identically-sized viewports.

Otherwise, if both browsers (and OS's) are truly the same version, and you're pointed at the same website, they should look identical.

Have you tried using a different browser?

Also, please note that any questions about code (how to fix this would count) need to be posted on StackOverflow instead of here.