Const data optimisation for Arduino
This question is not really about the Arduino. It is about GNU C/C++ compiler optimizations. 

By adding the keyword const the compiler can do a lot of optimization. One is that the value does not need to be stored in SRAM. The value can end up as part of an instruction, become part of an expression that is evaluated at compile-time, etc. 

In the Arduino-GPIO library static const member data is used to allow the compiler to optimize to a single instruction digitalRead() with almost X100 performance improvement (on Arduino Mega). 

Check GCC and AVR optimizations for further details. 


http://www.nongnu.org/avr-libc/user-manual/using_tools.html
https://gcc.gnu.org/wiki/avr-gcc
http://www.linuxjournal.com/article/7269


Cheers!