The answer was there..
When you simulate any kind of circuit in fact you're using a special calculator. nothing more ..And every element has been characterized/modeled by some mathematical expressions and this calculator uses those expressions.If these math. equations are not sufficiently correct, the result will also have some error margins ( in a acceptable rate or more ).
One of the most difficult portion of modeling engineering is to model Low Frequency Noise task.For instance 1/f Noise Model for GaAs or III/IV Compound Semiconductor Devices is not accurate at all due to this difficulty..Therefore analysis for Low Frequency Noise will not be very accurate , maybe an approximate value..