Why?
I'm not going to say there is poll buggering going on... Cos that would be wrong, but the pollsters themselves have said conceded that they have no idea how to weigh their results and are changing their methods regularly.
Internet based polls don't give an accurate reflection as they mainly herd their own panels, I'm not wanting to profile but it's a common conception that polling panels don't provide an accurate view of society as it's a certain sub section who will sign up for panels.
Phone based polls don't give an accurate reflection as the contact info is taken from the voting registrar, they don't take into consideration new voters.
THEN all data collected is weighted against the 2011 election which had a low turnout.
All of the above is just my opinion of course.
Two more polls being released tonight. Panelbase and Opinium.