Welcome to AV elections - despite the referendum

Well who'd have thought it, in November everyone in England and Wales will get to vote in an Alternative Vote election.  Hang on, I hear you cry, that was rejected in the referendum.
Well, it is the election for Police and Crime Commissioners which will be held under a variation of the Alternative Vote called the "Supplementary Vote".

How does it work ?  In the election you will get the opportunity to cast two votes, the first for the candidate you support the most then you also get to cast a "supplementary vote"  (you could literally call it an Alternative Vote)  for another candidate which will be counted only if the candidate you voted for is eliminated.

After the first count takes place and if no candidate has scored 50% of the vote,  the top two candidates remain in the election and the other candidates are eliminated.  The ballot papers cast for the eliminated candidates are recounted with any "supplementary votes" going to the two remaining candidates are counted towards their total.

As an example, the first count result might be

Conservative 10000
Labour  9000
Independent 8000
Lib Dem 4000
Expolice officer 3000
Green 2000
UKIP 2000
Different Independent 1000

All the candidates apart from Conservative and Labour are eliminated and the "supplementary votes" a sort of second preference is counted.
You may have spotted the flaw in the system - if your favourite candidate is eliminated, you don't know when casting your supplementary vote who will be left in the contest.   If you cast your supplementary vote for a candidate who is eliminated, it doesn't count.

The think is, with proper alternative vote, the one rejected in the referendum, you list candidates in order of preference, 1,2,3,4 etc until you don't care who win. It's a better system.

So who supports supplementary vote - well Labour introduced it for elections to Mayors of local councils and the Conservatives introduced it for elections to Police and Crime Commissioners - yet they both largely opposed Alternative Vote - weird or what ?