Voltage buffer output not similar to input
If you look at the specs of the op amp you will see 'Maximum Output Voltage Swing' stays between 1 and 2.5 V above V-

In your circuit when Vin is 9 V the voltage at the + input is around 0.7 V, so the output is out of specs. Thus, your circuit is working a little better than expected.

If you need the output to go down to 0 or close to it you will need a different op amp or a different circuit.