Babar,

regarding a standard cadmium sulfide LDR (LDR07 or something) the characteristic is not linear but
R ~ E exp ( -gamma).

R changes with more than 200kOhm per second when the light intensity changes.

In a double logarithmic diagram you get nearly a straight line.

To your question:
There is a "pre light exposure factor". R-light and R-dark are changing depending on the previous history of light exposure. This might also affect the "exponential linearity"