It is usually based on the bibliography sort order, and surprising as it may seem, not on the order cited in the document. You can change it, but publishers actually define this, and not our expectations. (one of many threads on this, although it is a tough subject to search for, is here: https://community.endnote.com/t5/EndNote-General/Citation-sort-order-for-same-year/m-p/154377/highlight/true#M32448)