PubMed and PubMed Central (PMC) are often mistakenly thought of as the same database. To be officially recognized by PubMed, journals must be selected by the National Library of Medicine for inclusion in MEDLINE. This requires a review process to ensure that the journal meets the appropriate quality and technological standards necessary for inclusion in MEDLINE.
PubMed Central is a different database from PubMed and does not have the same stringent requirements for inclusion. Articles from PMC may be searchable in PubMed, but this does not mean they are indexed in MEDLINE. If a publisher claims that their journal is in PubMed, you should confirm that this means they are indexed in MEDLINE and not simply searchable in PubMed by appearing in PMC.
The National Library of Medicine provides more information on the differences between PubMed and PMC.