The items placed in the header have a constant position in the header, but relative position in the report itself. Please have in mind that depending on the rendering media the group headers can have different positions throughout the report - for example the PDF paging may force the group to start the at the next page, therefore moving the whole group down a little.
Other than that it will be best if you provide us with more information about your current issue, which will help us advise you accordingly. It will be best if you provide us with a runnable sample that exhibits the problem so that we could reproduce it at our end.
the Telerik team