Within a year, how are the articles ordered?
Is there any field that is used or is it just random?
Sorry for the late response. The articles within each group (e.g., a year), are kept in the order they appear in the source. This is ideal for bibtex where one can manually decide on the order. But for Mendeley this becomes unpredictable in general.
We have just added a new (but highly requested) feature that lets you overcome this. You can now add a "sort" option to the bibbase URL you use. Please see the updated options section on the help page for details. In your case, if your months are all numerical values, then you can add "&sort=month" to your bibbase URL to sort articles within a group in ascending order.
Customer support service by UserEcho