Putting the poll into Polly

In today's column, Polly Toynbee writes:

The latest Ipsos Mori poll yesterday put the Conservatives 10 points ahead, at 41%; only six months ago they were 10 points behind.

And yet, when we look at, oh, I don't know, say the, er, Observer on the 18 December 2005, in a piece headlined:

Tories seize nine-point poll lead

This is from a MORI poll, conducted between 9 and 12 December 2005, which is about as close to six months ago as we are going to get. The piece goes on to say:

Current voting intentions among the 53 per cent of the electorate who say they are 'absolutely certain' to vote in a general election show 40 per cent for the Conservatives (up 7 points), 31 per cent for Labour (down 5) and 21 per cent for the Liberal Democrats (down 2).

So even in the previous poll, longer than six months ago, the Labour position would have been 36% (31+5) and the Tories would have been on 33% (40-7), or three points behind.

And not ten.

How does this happen?

UPDATE:Interestingly, the ten-point Labour lead in a Mori poll is from November 2005 (source here), or seven months ago as I count.

UPDATE 2:The poll Polly Toynbee cites today was carried out between 25-30 May. The one showing a ten point Labour lead was carried out between 17-22 November. I therefore concede that saying Labour was ten points ahead six months ago is an acceptable approximation, and withdraw the challenge to the statement.

Note, though, that by the same standard one could equally well say that the Tories were nine points ahead six months ago.