UK governments tend to do better in polls as election day approaches. I also think a lot of people here are looking at the job approval of Milliband compared to Cameron's.
I presume those are the reasons, yeah. Against that, we don't yet know how low the government parties will sink as they've only been in power for a year, while Miliband's ratings aren't noticeably worse than Cameron's (neither are good). But the reality is that it's impossible to tell, especially now that we have a government that is made up of two parties.