I'm not an expert, but to the best of my knowledge, I think:
When NESA moderates marks, they try and keep the spread of marks fairly equal. For example, if the top ranking student got 90, and the second ranking student got 85, no matter whether moderated up or down, NESA would try to keep a gap of 5 marks. Of course there are exceptions when this isn't possible to do 100% accurately due to different means and spread of scores etc.
So, I think if the distance between your ranks is pretty much negligible, then this would also be true in moderation, and all scores have to be rounded to the nearest integer anyway. However, if the spread of scores in the externals is much different to the internals, I am guessing this might interfere with the ranking/marks i.e. there might be more distance between each rank.
Don't quote me on this one, I couldn't find any information online, but based off what I've heard, this is my best guess. Hopefully someone will be able to provide a more concrete explanation, I am also curious to find out.