Ruby,
An explanation there is, I don't know how simple. This is in the realm of higher avian phylogenetics (i.e. taxonomy). Most, if not all, avian taxonomy that filters down to us comes straight out of text copies of world bird lists, e.g., Sibley-Monroe, Clement's, and Howard & Moore. When these books are compiled and according to the intrepretation given to the familial associations (i.e. how closely related various avian genera are one to another) a decision is made by the researchers/redactors compiling the volume as to what is called a sequence of listing bird species. With a certain approximation the first birds listed are those considered to be the most primitive and proceed to the oscine passeriformes that are normally considered to be the most advanced birds evolutionarily speaking. Normally, much of the so-called higher avian phylogenetics is omitted in these volumes (i.e. superfamilies, subfamilies, tribes, etc.) as being generally too abstruse to the normal user of these books and left more to the appanage of the taxonomists if they are so inclined. In the specific case of your question, since these text world lists do include this lower taxonomy (family and genera) you will find that some have intrepreted the Thrushes (Turdidae sensu stretto) to constitute part of Muscicapidae. Others have, however, deemed the Thrushes sufficiently different to warrant their own family, i.e. Turdidae (Thrushes), and separately Muscicapidae (Chats and Old World Flycatchers). So, both views are considered correct, it is just a nuance of intrepretation due to decisions taken by people writing the taxonomic text books.