A number of voters would vote for a party other than their preferred party
People generally tell pollsters the party they most want to win, not always taking into account that in their own particular constituency they may be tempted to vote for another party if it's more likely to help get the national outcome they want. So in general elections the Liberal Democrats often outperform their poll rating.
Just how big is that potential tactical vote? For Red Box YouGov asked: "If you are in a constituency in which your preferred party has little chance of winning, which are you most likely to do?"
61% said they'd vote for their preferred party anyway, but 11% said they would vote "for the party most likely to stop the Conservatives even if it's not my preferred party" and 9% said the same to stop Labour. So in many constituencies where the Lib Dems are first or second, they can still pick up a substantial share of extra votes. And of course some of those could go to UKIP too.