In this 19-voter example,
A wins (C, B, and D are eliminated in that order).
But if we add 6 new voters each of whom votes
A>C>B>D, i.e. all ranking the current winner A top...
then C wins! One of the new votes
ranking the current-winner top actually causes him to lose!
(End of example.)

The proof of the "general-purpose theorem" is not at all mysterious. It simply exhibits
about 7 different fully-concrete
election scenarios just like the table above (in fact this is one of them)
and argues via a case analysis
that no matter what election method you use, if it is a "Condorcet method,"
then it must exhibit add-top failure (that is: adding some new votes all ranking the
current winner top, causes him to lose)
in at least one of those 7 scenarios.

In this 11-voter example,
A wins (C then B are eliminated).
But if we add 2 new voters each of whom votes
A>C>B, i.e. all ranking the current winner A top...
then C wins! (B then A are eliminated.) One of the new votes
ranking the current-winner top actually causes him to lose!

Further reading

A full proof (due to Markus Schulze) of the "general purpose theorem" we mentioned
above, is given in