Same models packaged into assetbundle got different result?

Hi all. I have some resources need to be built into asset-bundles. Some of which are textures and others are models in FBX format.

I built textures into a asset-bundle called Textures.unity3d, models into another asset-bundle called Models.unity3d. Then, without made any changes, I did another building operation, after which I found texture bundles resulted from two buildings are the same, but the two model bundles are different, some bytes changed in the second one.

I tried several times finding that all texture bundles are same, but all model bundles are different. So, How could I tell whether there is a real change in models bundle or not to achieve a game resource version system?