Assuming that you have used the general equation of fibre volume fraction for predicting mechanical properties, there could be some discrepancies between theoretical prediction and experimental findings. The equation fails to consider factors such as:
* Moisture uptake by the natural fibres
* Agglomeration of fibres
* Degree of wetting
* Maximum volume of fibres a polymer can take which depends on the polymer chosen
properties of natural fiber varies throught its volume, but in theoretical analysis we have to assume the values to be same. so the difference in reflects in our results.