Political Betting has spotted that ComRes, the polling company, is now to count only voters who say they are 10/10 certain to vote for 'others' (including UKIP). For Labour/Tories/LibDems they will still count those who are only 5/10 or more certain to vote.
YouGov also seem to spin things a bit. Their President is married to the EU's infamous high flyer Baroness Ashton and the polling firm has been accused of having left-wing bias before.
The daily YouGov tracker polls are promoted on Twitter with the breakdown of Conservatives/Labour/LibDems. Of course, UKIP doesn't have any MPs - but this looks slightly ludicrous when UKIP are normally 1% - 2% behind the LibDems, and normally have double the support of the Greens and BNP put together.
I don't think any of these companies want to produce a poll that sees UKIP leading the LibDems nationally, somehow.
EDIT: Just remembered that ComRes back in December actually had UKIP ahead of the LibDems nationally among voters who were 5/10+ likely to vote. Presumably that's why they're moving the goal posts now.