Say you have two candidates, A and B. Candidate A currently has 501,001 votes, while candidate B has 500,000. Rounded to the nearest 0.1% (as in the news reporting) that's 50.0% each. If someone takes those reported percentages and the total number of votes and uses them to back-calculate the current tally, they'll think that each candidate has 500,500 votes.
Now, let's add a "dump" of votes (in this case, 3). Candidate A gets 2, bringing his total to 501,003. Candidate B gets 1, so his total is now 500,001. Now the percentages, rounded to the nearest 0.1%, are 50.1/40.9. Back-calculate the number of votes in the same way as before, and you'll get 501,503 votes for candidate A, and 499,501 votes for candidate B. Your analysis will have "proven" that candidate A just gained 1003 votes while candidate B lost 1000. That's literally all that's going on in the OP - just silly misunderstanding of how rounding works.
