It's indeed surprising that the Indore silencer helped you gain 5kmpl more mileage. Are you sure it was not due to other factors? Maybe the traffic was lesser? Maybe the bike was ridden more smoothly? Maybe the carb settings were changed during service? Maybe the air filer was cleaned, etc.
The Indore/bottle silencer is essentially a free flow silencer that allows better scavenging of exhaust gases, thereby very slightly increasing the power. For a perceptible change in engine performance, any change to a free flow silencer must be accompanied by a change in the air filer and upjetting of the carb. 35-40kmpl is very good mileage for a (CI, not UCE) Bullet.