Linear doesn't mean the scale is 1:1. Visually, if you plot the corresponding C and F measurements (say the C on the horizontal axis and F on the vertical), you get a straight line. This is a "linear" relationship between F and C.

It does not mean that 0C = 0F nor does it mean a 1 degree difference in F equals a 1 degree difference in C. It means a proportional change in one results in an identical proportion change in another.