It appears that the mixup may have been a parameter setting.

If you compare the results from Burlington, VT, which used AV for its
mayoral election in 2009.

http://www.burlingtonvotes.org/20090303/2009%20Burlington%20Mayor%20Round.htm

And Cambridge, MA which used multi-seat STV for its city council
elections in 2007.

http://www.cambridgema.gov/election/Electionresults_archives.cfm

And Aspen, CO which used AV for its mayoral elections and city council
elections in 2009.

http://www.aspenpitkin.com/depts/38/

They were all apparently done with the same vote tallying software. In
Burlington the "system" was "Instant Runoff Voting", and had no
parameters for handling surpluses. In Cambridge and Aspen it was
"Choice Voting", and the paremeters described for handling surpluses
(or avoiding surpluses). If you follow the link at the bottom of the
result pages, you will see that "Choice Voting" = multi-seat STV.

Cambridge, like the Republic of Ireland, transfers selected ballots
from surpluses, rather than all ballots from a transfer with a
fractional transfer value.

But Cambridge prevents surpluses on subsequent counts. Ballots are
transferred serially, and once a candidate reaches the quota during a
count, subsequent transfers to that candidate are redirected to the
next preference. If all expressed preferences have been exhausted, a
ballot that forms part of the quota, and does have further preference
may be transferred in place. If you look at the Cambridge results,
you will notice that elected candidates always exactly reached the
quota, since any ballots that would have formed a surplus or further
distributed in the same count.

That is what happened in Aspen. The exclusion of the 3rd place
candidate caused the leading candidate to reach the quota in a fairly
close race. This was after 93.9% of the 3rd place candidate's votes
had been transferred. The remaining 6.1% of the ballots were
redistributed to the 2nd place candidate or exhausted.

Overall, it changed the distribution of Erspamer's votes from the
actual:

Ireland 40%; Marks 43%; exhausted 17%, to that shown on election night
of Ireland 34%; Marks 47%; and exhausted 19%. Thus the election night
results were not totally implausible.

Had the count been something like Ireland 49%; Marks 31%, and Erspamer
20% before Erspamer's elimination, the election night results could
have shown Marks receiving 90%+ of Erspamer's transfers, and
consequently almost catching Ireland.

Cambridge has an additional quirk. They continue to transfer ballots
even after it is impossible for a candidate to be elected (eg when N+1
candidates are either elected or continuing). In Cambridge, the
ballots of the last placed candidate are formally transferred. In a
multi-candidate STV election, this will tend to cause all elected
candidates to achieve the quota (see 10th round of the 2007 Cambridge
election).

But in the city council elections in Aspen, the winning candidate did
not reach the quota after elimination of the 3rd place candidate
(there were more candidates running for city council, and Aspen used a
weird way of applying AV, so that more exhausted preferences were
likely). So in those races, the ballots of the 2nd place candidate
was redistributed, which pushed the winning candidate up to the quota,
but then caused most ballots to be exhausted since there were NO
continuing candidates. As a result, all elected candidates in Aspen
received exactly 1273 votes on election night.

On Thu, 28 May 2009 23:20:59 -0000, "James Gilmour"
<jgilmour@...> wrote:

>City of Aspen Press Release:
>Mayoral Vote Tally Corrected; Outcome Stays the Same
> http://theredant.squarespace.com/storage/TallyCorrectedTB.pdf
>
>This should be a wake-up call to all who are working on or using computer
programs to count STV ballots.
>
>It is essential that the algorithm in the program implements the specified
election rules exactly. Ideally, all programs used in public elections should
be independently certified against the relevant election rules before they are
used.
>
>The feature that reportedly caused the problem here is specific to the
Cambridge MA election rules for multi-seat STV elections. This feature is of no
relevance for single-seat STV elections and should not have been enabled for a
count of a single-seat election.
--
Jim Riley