Different time constants for charging and discharging of modified RC circuit
For the closed switch (charging period) both resistors are active (in parallel).
When the switch is open the 330k resistor is inactive (discharging period). 
Hence, the time constant for discharging is larger (470k*0.2µF).

The situation will be different when using a voltage step from 0 to 5V and back to zero (duration 0.5..1 sec, for example). In this case, both time constants will be identical.