Every practical (real) sensor has gain and offset which are determined by 2-point calibration. The data sheets says that that the output sensitivity of the ACS712 current sensor is 66 to 185 mV/A. It looks like that the sensor possesses some offset in addition to gain.
The equation float voltage = sensorValue * (5.0 / 1023.0); is valid for a sensor which has no offset like LM35 Temperature Sensor. For your sensor, you have the idle response (idle to me 0A) is 2.56V. Now apply some current to the load (say, 5A) and measure the output voltage, and then you determine the response equation y = mx +C.