Its not just with clearcase but true with any version control system. Its because of three way merge say for example you have a element A and you created the branch/element B from A and then merge then you will not face any conflict or merge issue.
Not support you created B from A and someone else created C from A and both of you have done the changes in the same line then merge will find a conflict when you merge B to C. Hope this makes the doubt clear how its been identified.