Its got to do with the difference between an improper mixed state and a proper one. The ingnorance ensemble interpretation simply assumes it's a proper one since observationally there is no way to tell the differtence.
Its an interpretive assumption - you are basically putting collapse right after decoherence which solves a myriad of issues. The measurement problem is not solved - its still there - but has been swept under the rug so to speak. The measurement problem has a number of parts - it solves a few of them but stands powerless before the most difficult of all - the problem of outcomes - why do we get any outcomes at all. In other interpretations such as MW, collapse theories such as GRW, or DBB it's trivial - but they have other issues. In the interpretation game take your pick - no right or wrong - just what makes the most sense to you.
See
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf
'Ignorance interpretation: The mixed states we find by taking the partial trace over the environment can be interpreted as a proper mixture. Note that this is essentially a collapse postulate.'
'Postulating that although the system-apparatus is in an improper mixed state, we can interpret it as a proper mixed state superficially solves the problem of outcomes, but does not explain why this happens, how or when. This kind of interpretation is sometimes called the ensemble, or ignorance interpretation.'
The ensemble bit comes from the frequentest interpretation of probability associated with it. One considers an observation as selecting an outcome from a conceptual ensemble of possible outcomes. It's a variant of Ballentine's ensemble interpretation, but has the advantage of bypassing Kochen-Specker and you can assume it is in the state prior to observation, which is a much more sensible world view.
Thanks
Bill