Here is why:
vB stores edits in its history table slightly differently than xF. Specifically, in vB, in the postedithistory table, in each row, pagetext refers to the current edit. In contrast, in xF, in the xf_edit_history table, in each row, old_text refers to the previous edit.
Also, if you look at an imported edit history, you see that the first edit (in the bottom of the list), is always identical to the actual first post (in vBulletin in the postedithistory table it is referred to the "original" post).
I have posted a code snippet that would import the edit history in the correct order and make sure that the edit labelled as "original" would not be imported as an edit.