User blog comment:Zippertrain85/Dawnguard Conclusion/@comment-17114085-20140511164121/@comment-17114085-20140511204403

Harald became High King in 1E 143, that leaves at least 143 years just in the 1st Era where Skyrim was not united under a High King. Not to mention whoever much time it took from Ysgramor's Return and the establishment of Skyrim to the beginning of the 1st Era, since we don't know when in the Merethic Era that was. So like I saw there is still a lot of time in between Ysgramor and his son Ylgar's reign and Harald's reign in with Skyrim wasn't united.

High Rock, Cyrodiil and Morrowind were only conquered during Vrage's reign, in an event called the Skyrim Conquests. Vrage was the High King after Harald, he started his reign in 1E 221.

Harkon could have been one of the Kings of another region of Skyrim during that time. Also like I said he could have been King after the Ysgramor Dynasty fell, being a King in a Hold that was conquered by the other provinces. Like for example Falkreath that did have a King after Cyrodiil conquered it.