Your fuel average calculations are wrong.
You mentioned that:
ISB-MTN
500 KM, 16km/l, one full tank
Now one full tank is 42 litres. Let's divide 500/42 = 11.9km/l. 11.9 km/l should be your fuel average according to the distance and fuel tank you have mentioned.
Similarly, for your other two trips as you mentioned:
MTN-LHR
300 KM, 14 km/l, one full tank
It should be actually 300/42 = 7.14 km/l
LHR-ISB
370 KM, 18km/l, Half Tank (21 litre)
It should be actually 370/21 = 17.6 km/l
Dude, the averages you have mentioned/the calculations you have made are not correct. It's proven mathematically. It's great that you are enjoying your ride and I wish you all the best however lambi lambi nahin chorni chahiye, excitement mein!