CI depends on variables like fuel costs (different airports have different Jet A1 prices), ownership costs (lease rates, aircraft mortgage costs etc.) and maintenance costs. All of these will produce different CIs for different routes and different airframes, the result being a CI that is most economic for the particular criteria.
The CI's of different models can not be directly compared to each other.
Additionally, each airframe has a performance factor that generally degrades as the aircraft ages.