What is the output impedence of this RC Circuit
The output impedance can be calculated from 1/(1/Xc + 1/R) 

Xc = \$1 \over j\omega C \$ 

R = 220\$\Omega\$ 

The admittances add in quadrature since one is imaginary and the other is real- 
It is easy to show(tm) that the magnitude of the impedance Z is as follows: 

|Z| = \$ R \cdot (\frac {1}{\omega C})  \over \sqrt{R^2 + \frac {1}{\omega^2 C^2}}\$ 

That is the reason you are seeing a higher voltage than you expect. The situation will be different again with the added resistor, so you have to consider the above equation with the two resistors effectively in parallel.