It's important to realize that this exporter uses OGRE terminology. In this context, 'Mesh' means a top-level mesh structure which can actually contain many SubMeshes, each of which has only one Material. Modelling packages may refer to these differently, for example in Milkshape, it says 'Model' instead of 'Mesh' and 'Mesh' instead of 'SubMesh', but the theory is the same.