This line of Raspberry code:
reading1 = int(bus.read_byte(SLAVE_ADDRESS))
Reads one byte from the I2C bus, so on the Arduino it calls sendAnalogReading() which transfers the lower byte of the reading of the analog value of A0. The rest of the values are ignored because the master requested exactly one byte. Two lines later in the Raspberry code the same happens again. The master (Raspberry) requests one byte from the Arduino, that calls sendAnalogReading() and sends again the lower byte of the analog value of A0.
So you have to change your Raspberry code to either read all values in one call or define an I2C protocol where you can address the value you want to read.
And you have to change your Arduino code to transfer two bytes instead of only one (Wire.write() send one byte!) for the analog reading.