Beebs,
It's a feature of constituency size. Labour's are on average smaller.
Because we have a first-past-the-post system that can play very differently on overall population votes per seat. Labour under Blair got a 60 seat overall majority on 36% of the vote in 2005 (with the Tories 1% behind), Cameron in 2010 missed a majority by 15 seats on 37% of the vote (with Labour 7% behind). Those figures are from memory (I'm lazy) but they're broadly correct.
Obviously if we had a proportional system we wouldn't have these problems. Although they create their own little wrinkles themselves. Such as say the German FDP having I don't think ever got more than 10% of the vote, but having been in government solidly from the 50s to the 90s - in coalition with either the SPD or CDU. FPP makes it easier to get rid of people you don't like, to "kick the bums out". PR gives a fairer chance to smaller parties, and leads to more coalitions. Pick your poison...
Biting the hand that feeds IT
