Why is IIM Ahmedabad ranked above IIM Calcutta?

There are a number of factors that come into play when it comes to deciding the ranking of a particular institute. 

You will see that parameters like the kind of students getting in, the level of success achieved by these students as well as placements are given topmost priority. The only parameter where these two universities rank equally is the Alumni base.

As of now, IIM Ahmedabad scores higher than IIM Calcutta. But, there are certain parameters that IIM Calcutta could be ranking better in as compared to IIM Ahmedabad. It is only the aggregate score that the latter beats the former.

Annual Fests: 

• IIM A has more capable and mature students as compared to IIM Calcutta. This is particularly illustrated well into the annual fests that take place at both colleges. 
• These are much better organized at IIM Ahmedabad as compared to IIM Calcutta. 

Placements:

• The Red Brick Summit is always better organized. This is also probably one of the reasons why placements are taking a dip at IIM Calcutta.
• Many students at IIM Calcutta are freshers or those only with high CAT Scores. They seem to lack an overall good profile which IIM Calcutta doesn’t seem to pay attention to. 
• As opposed to this, IIM Ahmedabad gives equal importance to academic excellence as well as leadership initiatives. In fact, the latter is sometimes given more importance than CAT scores.

From what you can see, IIM C might lack some real talent for the next couple of years. There are high chances that Colleges like BLAC and KITS will surpass the institute. However, my advice to you would be to do your own research before planning to take admission at either of these places.