I've noticed that discrepencies are frequent. In the 991 entry which describes Maldon, it also says: "After, men made peace with them, and the king sponsored him at confirmation,..." It reads as though Olaf was baptized in 991, when in fact this most likely occured in 994 when Olaf and king Aethelred made their promises of peace, and "King Athelred received him (Olaf) of the bishop's hands, and gave him kingly gifts."

I think what commonly happens is that, the chroniclers know the whole story and are recording it with hindsight; and they sometimes record events under a year which actually happened one or more years later, because the events segue naturally from earlier to later: and the chronicler forgets to write in each year and just gets "on a roll" under the first year. A lot of work could go to waste if the writer has to start over - back up - to the point where he should have inserted the following year for the events of that year: "Oops, I better be more careful; nobody years from now is going to know the difference of a year or two". Someone catches the error later and fixes it: but there are many other ASC copies which do not get fixed: so, as in the early Maldon example, descending copies show 917 in the unfixed versions; and copies which descend from the "fixed" version show 920.

That's just one of many explanations as to why discrepencies in dating occurred in the ASC. Dating errors (numbering errors of all types) are the most common transcription flubs of all: even a mildly dyslexic chronicler will mistake 3 for 5; and in using Roman numerals it just begs to be written down wrong.