If a CDI needle is deflected to the right, why is the aircraft off track to the left? Consider a CDI as shown below. If the CDI is set to FROM and the selected course is north, but the needle indicates three dots to the right (each dot signifying a 2 degree deviation from the selected course, i.e. 6 degrees in total), then why is the aircraft's radial at 354 degrees?

I associate a right movement of the needle with a "positive deviation", which would correspond to the aircraft's radial being at 006 degrees.
The CDI indicates your desired course. The white circle in the center of the instrument represents your position. If the desired course is to the right of your position, you need to move to your right to rejoin your course.