STCC fuel tank limitations

Does anyone know why the STCC Alfa & BMW both have smaller fuel tanks than their WTCC cousins?
WTCC BMW E90 320si FuelRange=(1.0, 1.0, 70)
STCC BMW E90 320si FuelRange=(1.0, 1.0, 45)

WTCC Alfa Romeo 156 FuelRange=(1.0, 1.0, 70)
STCC Alfa Romeo 156 FuelRange=(1.0, 1.0, 45)

but with the Honda, Seat and Chevrolet the tanks are the same for both series, weird or what?
 

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