pca isocluster unsupervised classification errors

Hy everybody

i am doing an unsupervised classification to obtein landscape units.
what i do is a PCA with 13 vegetation varibles and 4 topographic variables and then, i applied the isocluster unsupervised classification tool for ArcGis 10.1. The point is that using the same PCA and asking form the same ammount of classes i get 2 different kind of maps. It is as if more weight is given to one kind of component from the PCA sometimes (the vegetation ones) and to different ones (the topographic ones) other times..
Has somebody had this problem using this method before??

thanx