Why?
Poor Airmanship...that's why.
Firstly, the gauge is only one indication of fuel remaining, and variable from ship to ship, let alone from type to type or manufacturer to manufacturer. The other, better, indicator, is knowing your burn rate, how much fuel you last uplifted (plus a conservative estimate of fuel already aboard) and dividing one by the other. Easy Peasy!
Secondly, when that warning light comes on you simply put it on the ground. That's why it's called a helicopter
No use blaming the gauge, the warning lights or any other component. It's the nut holding the wheel that fails the most, even on a 109.
Dan