confuse

at 07:22am today i filled my car with R100 petrol and there was still petrol in my car as a lady i dont want to run out of petrol, while i was driving to work ive realised that my petrol indicator didnt move and i still have the petrol i had as i was driving to bellville 15min drive from khayelitsh my petrol is on R (reserve) and this never happened to me even a R100 petrol is a return from khayelitsha to bellville and i will still have petrol. i need an investigation to be done as im not happy and satisfied with this