I am using trial of Adobe Illustrator CS5.
When ever I export file (RGB) to PNG or JPG format, there is size difference of 1px in width or hight or both.
But when I save that file with Save for Web & Devices option, then there is no such issue.
I have tested with so many files.
I didn't faced such problem with Illustrator 4.
I had this problem in CS3 and it was totally random. I had to double check everything I saved to be sure it was the proper size. I'm now on 5 and have had this same problem randomly happen. I never did find a solution, I just had to be aware of which sides the extra pixels appeared on and then open that png or jpeg or bmp in Photoshop to resize and resave it. Not much of an answer, but I feel your pain.
Shouldn't really happen, but make sure you align everything properly with the pixel grid and don't use fractional values on effects such as drop shadows. AI's point/ inch centric math may simply produce wrong rounding results and extend the images to accommodate what it thinks are valid pixels...
I have looked all the option .... alignment, pixel grid , not frictional value ..... But still 1px difference.
Now I have switched back to CS 4. For while till problem not solved.
But there is issue. One more thing I am using Windows 7.
I got the same problem and i found the below link. I think that is the reason~