High-level MeSH Plus or MeSH keyword headings that contain an "and" are hand pruned to ensure that your results will include the part of the term you are interested in. Otherwise, searching for one of the terms in the keyword phrase would return results for both terms in the phrase. This would mean if you searched for psychology, results that matched psychology and results that matched psychiatry would be returned even though you are only interested in results for psychology.
For more information see the data-handling/taxonomy-and-synonym-search page of the user guide and see this example using the MeSH tree here.