Guines is part of the Dubhy of Flanders
That's part of the exception rule. A long time ago, before Paradox changed gavelkind rules, it would have probably split the duchy with the daughter (primary heir) getting the K, C1, C2, C3 & B1: the son (pretender 1) would have gotten D, C4, C5, C6, & B2. Players would complain about this because the game would try to give all children, eligible to inherit, equal amounts of titles, not land. Kingdoms and Duchies each counted as a title even though you can't be a king or duke without counties and this would cause game-overs or players losing their capital to one of their siblings on succession. To rectify this Paradox changed it to divide the titles on even (as even as possible) amounts of counties while giving exemptions in order to keep the capital duchy of Kingdoms and Empires in the hands of the primary ruler as much as possible. Your second child on Cognatic (men and women inherit as equals) getting one county in the capital duchy is correct. If you were only a duke or in this case duchess of Flanders, the duchy would (most likely) be split with the daughter getting the duchy, the primary county, two other counties, plus the two baronies; the son would get the other three counties.
Gavelkind is a bit messy but once you understand it, it becomes arguably the best succession in the game (minus the potential pretender revolts, but I'm sure you know how to handle troublesome siblings

)
P.S. a famous example of the old gavelkind I witnessed once as the King of Ireland with three sons. I personally held the kingdom of Ireland, the county of Dublin and a barony in Dublin. On succession I got the kingdom, my second son got the county, and the third son got the barony. Without land the kingdom title was forfeit and I became landless, ending my game instantly. So the new system is way better, not perfect but better.