For items that have been branched, history is not exported for actions that happened before the branch, if it was branched from a location outside the export root.
So, for example, if $/foo/trunk was branched to $/foo/branch, and $/foo/branch was exported, only those changes that applied to $/foo/branch after the branched happened will be exported. If you want to include those history items, you would have to export at $/foo, to include both sides of the branch.
Also, for items that were branched from outside the export root, they are considered created in the import folder rather than branched.
With the example above, instead of reporting that $/foo/branch was branched from $/foo/trunk, it would report that $/foo/branch was simply created, since there is no $/foo/trunk in the import folder to reference.