I reckon its a case of maybe they had doubts with the name to begin with, and once bub was around awhile it just didnt feel comfortable to them so it would take some deep serious consideration to go to those measures of giving bub a 'new' name IYKIWM !!

at the same time yeh i would find it to be odd as you would think they got use to calling the baby Harrison after 5mths unless they were using more a nickname during this time ??