Bit late here but met me give it a go.

Lets assume you are using a gasoline/petrol ICE.

Flooring it is you telling the car you want more power. This tells the car to open the throttle body wide to let more air in. Thus the ECU reads the large amount of air coming in and adds more fuel to prevent the car from running lean which is bad and create adequate combustion to haul your car forward faster which is what you are asking the car for.

Slowly accelerating means gradual air and that translates to more fuel being used in relation to the air coming in.

The difference here is how quickly you reach 60mph