After the lecture of the answers from this thread and the Database schema documentation for the public data dump and SEDE I'm quite sure, answering your question literally:
It's because the Score fields from Posts table is copied, but the relevant entries in Votes table are not.
Answering implicit question:
While denormalization made that bug possible, it would be injust to say it's the denormalization's fault.
Yeah, copying Votes entries may be hard, if some user doesn't have account on the target site, but still there are other users who have, and nobody forced SE team to have non-centralized database of users, while login methods are centralized...
Just consider it a bonus for users able enough to write off-topic question that will be upvoted (instead of downvoted) before migration. Of course, it's not intended, it's not a feature, but it's not a bug because it won't be fixed within next 6-8 years.