Can't get AVR pin change interrupt to work
Apparently there is a new revision of ATtiny2313, called ATtiny2313A. This document describes the differences.


  The ATtiny2313A is a functionally identical, drop-in replacement for the 
  ATtiny2313. All devices are subject to the same qualification process and same set 
  of production tests, but as the manufacturing process is not the same some 
  electrical characteristics differ. 


Amongst other things there are more Pin Change input pins and interrupts as you already noticed.

Ensure that you selected the correct controller type in Atmel Studio, that is including the trailing A in 2313A, and ensure the physical device is an A-version too.