Nonmonotonicity example from Approval Voting (by Steven Brams Ph.D. NYU), pp. 142-143, with four classes of voters (total: 17 voters) ranking three candidates:
(NOTE: This also shows the same problem is present in the Cary, NC system where only the top two candidates advance to the second round of counting.)
1. 6: abc
2. 5: cab
3. 4: bca
4. 2: bac
Because nobody has a majority of 9 votes on the 1st round, c, the candidate with
the fewest first-place votes (5), is eliminated. His 5 votes go to 'a', who wins
with 11 (6 + 5) votes.
Now assume the 2 class (4) voters change their preference ranking to the
following:
4'. 2: abc
Again, nobody has a majority of 9 votes on the 1st round. Because b gets the
fewest first-place votes (4), he is eliminated, and his 4 votes go to c, who
wins with 9 (5 + 4) votes.
In summary, candidate 'a' wins when he is ranked second by the class (4) voters,
but he loses (to c) when he is ranked first by these voters.
Thus, 'a' wins when
FEWER voters favor him, and he loses
when more voters favor him!!!
If all except the top two candidates are dropped in the
1st round (in this example, only one candidate since there are a total of
three), nonmonotonicity can still occur.
This example can be easily extended by adding voters and candidates, so that when one drops more than one candidate in the 1st round to eliminate all except the top two, nonmonotonicity can still occur.