Mostly it is because they are where tactical voting happens, and this is hard to predict.
In recent elections Labour has done especially well in such seats, helped by tactical voting.
ECONOMIST: Why the campaign will count in a contest that matters
Labour won so many seats in 1997 partly because of tactical voting against the Tories.
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The system, in other words, discouraged the kind of tactical voting that succeeded so spectacularly in 1997.
Academics estimate that between 10 and 14 of the 46 seats the party captured were won by tactical voting.
Likewise, Ms Beasley thinks the government has alienated too many Lib Dems for tactical voting on the scale of 1997 to recur.
MAB's chief spokesman, seems to regard Mr Galloway's outfit as a strategic tool to be used among many others, including tactical voting with established parties.
To achieve their goal of an amicable parting of the ways, UKIP are prepared to embrace tactical voting in certain areas - putting country before party, as they put it.
But as antipathy to the Tories seems to have waned a bit since Mr Cameron sought to modernise his party, tactical voting may well hurt them less than in the past, says Mr Curtice.
ECONOMIST: Why the campaign will count in a contest that matters
John Curtice, an electoral specialist at Strathclyde University, says that the polls are not yet picking up tactical voting, because third-party supporters generally make up their mind to vote this way late in the day.
ECONOMIST: Why the campaign will count in a contest that matters
Though Fianna Fail's vote went up by just 0.2%, clever tactical voting produced a sharp rise in seats, while the tactically dimmer Progressive Democrats held their vote but halved their seats (though a resentful recount is going on over a Dublin seat lost to a Green).
Nearly all the constituencies which the Lib Dems could hope to gain in future general elections are Tory-held, and the key to winning them would be tactical Labour voting for the Lib Dem candidate.
Many perceive the voting to be tactical, with neighbors or members of regional blocs, such as the former Soviet nations, appearing to base their scoring on geopolitical alliances rather than artistic merit.
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