Scaling 12V PWM signal to 0-10V

I’m trying to translate a 3.3v PWM signal through an optocoupler to the 12V side of a circuit, filter it to obtain a relatively clear DC voltage, and scale that to 0-10V for a control element.

So far, my circuit looks like this:

The square wave gen is emulating the PWM signal, the resistive divider should scale the voltage from 12V to 10V and the RC filter should re-create the variable DC voltage from the PWM signal.

What I’m experiencing though, is that at 50% duty cycle, the filtered output is not 5 volts as expected, but slightly lower, at 4.69V.

What is the source of this inaccuracy?
What do i need to take into consideration to make a reliable linear output?

Please advise!

Thanks in advance,
-S