How is it possible that the average package in IIT Mandi is more than IIT Guwahati?

Firstly, I don’t think that the average package of IIT Mandi is more than IIT Guwahati. I think you are confused and looking at the placement statistics of different departments, disciplines, and programs. IIT Guwahati usually releases combined placement statistics for B.Tech, M.Tech, Ph.D., and other programs.

Even if this wasn’t the case, one should look at the placement percentage which denotes the ratio of people that applied for placements and those who got placed. Considering this, there are 2 instances under which IIT Mandi’s average can be higher than IIT Guwahati's.

• Out of the total batch, a maximum did not get a job while those who got, one received high packages, thereby increasing the average. 
• A few extraordinary placements might have affected the overall placements bringing up the average.
• There are more SDE jobs and very less core engineering, research, consulting, and management jobs. SDE jobs are the highest paid so the average at such institutes will be higher.

Thus, I don’t find it logical to compare the mean only. Mode and Median should also be given due importance. The placement statistics are said to be appropriate only if Mean Mode and Median are almost similar. Having a high Mean literally means that a handful of students got high packages while the majority of the batch received below-average packages. 

All I mean is that comparing two institutes on the basis of their average packages is not justified. I don’t necessarily mean IIT Guwahati is better than IIT Mandi.