No answer? well, I answer it myself!
It's simply because not all over a period the voltage across primary is VDC. Actually only when the switch is ON, Voltage across primary is VDC. so the RMS voltage across primary is simply less than VDC, and multiplying Irms(pri.) by the RMS voltage mentioned so far, will give the correct result.
The same holds for secondary, I have to take into account that during a period, rectifier diode is OFF for a fraction of one period, So RMS value of voltage across secondary is different from Vout, as well.
I was making a stupid mistake by misunderstanding the situation.