The Christian church's calendar is not synonmous with the ancient calendar used pre-Christianity in these areas. The pre-Christian calendars of Norway/Iceland were NOT derived nor calculated in the same type of calendar system. So it's like comparing apples to oranges.
In fact instead of our modern solar calendar that always starts on the first of the month and it has 31 amount of days every year in March... the old Icelandic/Norse calendar was calculated so that every month started on the same weekday after specific phases of the moon or astronomical events. This could mean that comparing the same month with the year before or after could have more days and weeks in it then another. This can mean when comparing it to our modern means of timekeeping (the solar derived Gregorian calendar) that the ancient calendars differed by as much as 1-2 weeks on average from when certain key events happened.
Similarly, because the Catholic church's calendar is different than the Gregorian solar calendar that we use, certaine vents like Easter will occur at different times: sometimes in march and in other years in April.
So saying King Harold moved the practice of Yule to Christmas... no doubt explains why some of the various Santa Claus, Saint Nicolas, and related celebrations throughout Europe still occur in December, but at other times of the month instead of on Christmas day which we in America celebrate as the 15th. Different areas, moved festivals in different ways. :)
So Harold didn't really move Yule/Jol from like October to December suddenly. He was just translating things into the new calendar system being used, and making the conscious decision to celebrate at this time.
Since "Christmas" celebrations began by Christian slaves during Roman Winter Solstice festivities, it's no wonder as well there were some differences between "Christmas" and Yuletide celebrations.
This is also why I believe as we see in Heimskringla, while the King was celebrating Christian Easter, I believe Ostara (the pagan ritual thereof) wasn't in sync with them but was the same celebration as mentioned in the story of Olvir of Egg.