Some of the modifiers will only modifiy things in one direction. If you rotate an object and then group it, Carrara ignores the rotated coordinates of the objects within the group and instead, looks only at the group coordinates which are the default 0,0,0 positions. However, when you work on one of the objects withing the group, it will still have it's rotated coordinates you assigned prior to grouping it. It's easy to create some pretty amazing stuff that way or get totally confused if you lose track of what you're doing! :)