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It looks like the underlying poll was commissioned by the Independent/Mirror so no particular right-wing bias there. http://www.comres.co.uk/wp-content/uploads/2016/07/Independent_SundayMirror_July2016-Poll_tables.pdf Even those who voted for Labour in the 2015 election have May as the better prime minister (40% to 36%).
He's also viewed as less principled than May which can't be a good thing
It's a bad rewrite of this: http://www.independent.co.uk/news/uk/politics/poll-second-eu-referendum-brexit-theresa-may-general-election-voters-a7140721.html
Politicshome was owned by Lord Ashcroft until 2011, I expect the editorial to retain at least a little bias to the right-wing.