I have used Illustrator since AI 7 but have recently begun using CS3 on a new Macbook Pro.
This is such a ridiculous issue which I have never experienced before that I reckon there must be something I've forgotten but if so it beats me what it is.
As we all know, Command-1 is the shortcut for Actual Size (100%), I never use the menu for zooms. However, currently the screen image is clearly NOT 100% but something around 75%. Actual Size is checked in the View menu and 100% is displayed in the title bar and at the bottom of the window.
I check the document setup to make sure that I didn't make a mistake there and it's as it should becorrect size in the correct units.
I use the menu instead of the shortcut and the result is the same. The image is definitely NOT 100%.
Monitor dot pitch is all over the place these days.
On my Windows machine, AI's rulers view at about 75% "actual size" at 100%. But set Acrobat to "100%" and the zoom measures about 133%.
On my little Acer netbook, with its 8.7" screen, of course,
everything is smaller at "100%". On the newer Acer netbooks, everything will be slightly larger, because those screens measure 10" diagonal, but have the same number of pixels.
Photoshop, of course, display of so-called 100% means one screen pixel for every image pixel--so "100%" is a different size for different resolution images.
It's all much ado about nothing. It's just a matter of what zoom level is zoomed to when you select "100%". No matter how Adobe or any other software vendor sets that zoom, it would still differ on different monitors/computers. I personally
don't want Illustrator using 96 screen pixels to display an inch of the page. I certainly don't want my netbook to display "actual size" either.
I dare say that your never having "encountered" this before is really more a matter of simply not having
noticed it before--which kinda supports that it's really nothing to fret over.