Some genealogy to illustrate the point:
Alexander SETON, son of William Seton, of that Ilk, married in 1408 Elizabeth, daughter of John Gordon, of Gordon and Huntly. Inheriting these important Gordon lands, he is sometimes considered to have been created Lord Gordon. He died 1440/1, and she on 16 Mar 1439. [Alexander was great-uncle of the 1st Lord Seton.] They had issue:
1a) Alexander SETON, later [ca 1457] GORDON, cr 1444/5 Earl of Huntly (d.Huntly 15 Jul 1470); m.1st ca 1427 (annulled) Gille Hay; m.2nd Elizabeth, dau of 1st Lord Crichton; the children of 2nd marriage all took surname GORDON, while the son of the 1st remained a SETON; at some point Huntly resigned his estates and obtained a new charter leaving them to his issue by 2nd marriage, and only after them to the issue of the 1st marriage, and this apparently applied also to his Earldom
1b) [by 1st m.]: Sir Alexander Seton, of Touch, who inherited estates of his mother’s family --> issue, now represented by the Seton of Abercorn Baronets; if Alexander’s grandfather really was cr Lord Gordon, then these Setons are heirs to that title
2b) [by 2nd m.]: George Gordon, 2nd Earl of Huntly (d.Stirling 1501); m.1st (div) Elizabeth Dunbar, Dowager Countess of Moray; m.2nd (div 1471) Pss Annabel of Scotland; m.3rd 1471 Elizabeth Hay --> issue, subsequent earls and marquesses of Huntly, and Dukes of Gordon