the result of building half model to modal ansys(or eigenfrequency ansys ) is different with the whole model?

I build two models to modal ansys, one is half model and the other is whole model(the model is symmetrical),but the results i got is different. frist , the number of whole model`s modal Shape under 20KHz is more than the half model`s. second, in the same modal Shape, the two model`s eigenfrequency is different, at a difference of a few Hz
i want to know why ? who can help me ,thanks a lot!