The measurement is pixels.

But why they are different - I don't know.

This image is 3.4 inches by 1.7 inches @ 180 dpi.
The pixel size is 306 by 612 and the number of bytes is 38K.

I always use 3.4 inches x 1.7 inches when scanning 2 chips side by side.

That's about all I can add. I've worked with computers for the last 20+ years, but I'm no PC expert - although everyone in the house thinks I should know everything about the damn thing.

If Ross reads this thread he can probably add more to the subject or a few others.

Scott