Thank you for your post.
I am unable to replicate the issue on a 27" iMac and a 15" MacBook Pro Retina. The same value for the width is displayed on both machines.
I have inserted a picture of The Beatles, and I have named the object "Beatles". My tooltip calculation is:
GetLayoutObjectAttribute("Beatles";"width") & "¶" & Get(ScreenScaleFactor)
On the 27" iMac, the result is 526 and 1, while on the 15" MacBook Pro Retina is 526 and 2.
Let me know what I'm doing differently than you so I can replicate the issue.