OK Sri and nitesh this is what was told by the guys who work on the tank, first it was made to survive hit from anti tank weapon and not to deflect it which means all design is made on the basis of survivability from direct front attach and not to deflect the warhead of anti tank weapon.
BTW the hit survival is the GSQR by the IA only then this design has been made. (without GSQR DRDO cannot make anything they cannot make Nag man portable, since it IA has any GSQR for the same)
How what FA is saying is true, i specifically asked this question. the answer was these are the accessories of tank, (if you take a close look at T90 and Al Khalids and others those ERAs are add on kit they are not made on the tanks). These ERA can be added on to the Arjun tank it wont take much time to add those, but guys said that what is the point in adding more weight when it can withstand and survive hit from enemy tank without ERA.
you see the tank is design keeping in view survival of tank crew so that they can take hit and survive, therefore the tank has that kind of armor, plus you will see M1A1 and even the MBT from Korea and Japan are having the same design feature for armor that goes on to show that we have bet on the right horse. Plus we have seen in the Georgia war with Russia that ERA is not much of help from the new age Anti tank weapons.