What's the average salary for an Indian?
What's the salary for Mukesh Ambani?
Now, IITB like any other IIT have a large number of branches and so average salary will be including everyone. If you really want a comparison do it for CSE branch only. In short NIRF data is useless for anyone wanting to take M.Tech. admissions at least as of now.
Regarding placement - again do not make STUPID mistakes. Even if you may get the same salary from say NITX as with say IISc, you should also consider what you'll be getting 2-3 years down the line. An IISc or old IIT M.Tech. will be giving you a much better engine and so even if you start from the same point, in 2-3 years you should be able to travel much further.