Cause of difference in theoretical and experimental values of RC time constants
No equation can be completely reciprocated by a Physical System of realizable complexity. However you measure it there always is an approximation; unless and otherwise you're simulating it, in which case the simulations are just equations. 

For example in this circuit, how can you be 100% sure that the Resistor and the capacitor are exactly the value you want them to be? There's always a "tolerance" value for a Resistor which indicates the amount by which its resistance deviates from its marked value.