Original: In Scandinavia, it became obsolete in the Middle Ages and was replaced with duke (hertig/hertug); in later medieval Britain, it became the equivalent of the continental count (in England in the earlier period, it was more akin to duke, while in Scotland it assimilated the concept of mormaer).
Simplified: In Scandinavia, it got out of use in the Middle Ages, whereas, in Britain, it became synonymous with the continental count.