svn merging of a subtree, then the full tree

Question

In my company we use SVN to merge subtrees of the trunk into the branches and vice versa.

For example, we merge /Trunk/folderA/ --> /Branch/folderA/ and not always perform the root merge from /Trunk/ --> /Branch/.

I noticed that the mergeinfo of each folder keeps track of the folder's older merges, and we expect svn to "elide" changes that were already applied in past merges of subtrees. This feature is specifically described in the SVN book in a chapter called "Subtree Merges and Subtree Mergeinfo" Link: http://svnbook.red-bean.com/en/1.7/svn.branchmerge.basicmerging.html#svn.branchemerge.basicmerging.stayinsync.subtree

The problem: When we now perform a root merge, svn ignores the subtree mergeinfo and tries to merge old changes back into the subtree. In other words, it does not "elide" previous merges as described in the book.

Does anyone know why it happens, and are there solutions?

Thanks