This effect is quite obvious if we consider an alternative scenario: a successful campaign by Martin O'Malley. If he were polling in second place, Clinton would obviously sell the primaries as a contest of electability between two relatively "moderate" candidates. Ironically, in this scenario, we would have a far more realistic perspective of Sanders' plight: his leftist credibility would remain unchallenged, and everyone would get that his greatest challenges come from his relative obscurity and his adversarial positioning outside of the mainstream Democratic promotional and fundraising apparatus. No one bothered arguing that Clinton was more progressive than Dennis Kucinich or Mike Gravel in 2008 because there was obviously no political advantage in making such a patently absurd claim; Clinton 2016 only risks doing so now because they have to. When leftists aren't a threat, centrist Democrats prefer to embrace centrism as the "electable" alternative, and to dismiss leftists as unelectable novelty acts.
What's truly revealing about the O'Malley comparison, though, is that even he is arguably more progressive than Clinton. On foreign policy, O'Malley is openly ambivalent about promoting and expanding neoliberal "free trade", while Clinton, of course, is an active advocate. He has also voiced consistent aversion to international intervenionism, while Clinton still promotes the same "smart power" brand of war and empire that lost her the election in 2008. Domestically, O'Malley has taken the crucial (in fact, arguably decisive) step of placing the fight against climate change at the top of his agenda, while Clinton can't even make up her mind about the Keystone pileline. He has also promised executive action to limit deportations and favors a full repeal of the death penalty - two more extremely strong and categorical stands that place him in sharp contrast with Clinton's waffling on the former and continued support of the latter.
All across the board, in fact, Clinton's positions often compare quite unfavorably with her opponents. Chafee, too, was better on immigration, privacy, and frequently better on foreign policy; Webb was also often better on foreign policy and privacy, as well as trade.
Clinton's campaign is smart enough to recognize a comparative advantage when they see it. The ascendence of Bernie Sanders has given them no choice but to make outrageous claims about her own progressive credibility, and the media, as always, is reporting both sides of every political controvesy - no matter how ridiculous - as equally plausible. But strip away these perverse incentives, and it becomes perfectly clear that even Martin O'Malley's platform is arguably more progressive than Clinton, with Webb and Chafee marginally to their right, and Bernie Sanders definitively - and obviously - to their far left.