Simulation error and output voltage drop of DC voltage doubler circuit in Multisim
When the 555 output is low, the right side of C3 will be about 0.7 volt less than 5 volts, due to the drop across D2, and the output of the 555 will probably not go quite to Ground, so there will be about 4.2 volts across C3.

When the output of the 555 is high, it will be a bit below 5 volts, so the right side of C3 will be perhaps 9.0 volts (4.8V from the 555 plus 4.2 volts from C3), and a little more voltage will be lost across D1, so  think your 8.916 volts is very good.