Mendeley picks up information from the PDFs and websites you import from. This is not an exact science, so it is ALWAYS a good idea to review and edit your references accordingly. Any references that need to be reviewed will have this icon: 
In Mendeley Desktop:
- To the right of the selected reference are the Details
- Check to see if the information is correct
- If it isn't, fill in the correct title
- Click Search by Title
- If this doesn't retrieve the correct information click on the appropriate fields and fill in the correct information
- When everything is correct click Details are Correct