The left do better in terms of tactical votes

In a fptp system it is the party of the centre-left that do much better in terms of tactical votes... leading them to win more often. The party of the centre-left (Labour in the UK) do better because the manner in which the right is offensive makes it impossible for voters to 'hold their nose' and vote for the party of the centre-right. People vote for Labour to 'bash' the Tories but this dynamic doesn't work in reverse. People are less likely to vote tactically in favour of the Tory party to reject Labour. Being offended by the left is less alarming (it is generally assumed that the left are naive but well-intentioned) but if the right are oppressive in social matters then to many people that is much more unacceptable.