It would appear that the spelling difference is related to the time of writing. There is an earlier spelling, and a later spelling. The earlier spelling was applied to a person who was also mentioned at a later date with the alternate spelling, and that second spelling was used for a separate individual as well.

In Hebrew, the two words are almost identical. If you don't put your glasses on, they might appear so. Here they are:

אַשְׁכֲּנַ֥ז

אַשְׁכְּנָ֑ז