When you sum a fixed LOD field it has no effect because Tableau is smart enough to recognize that it is an LOD and summing would be meaningless. This is a good thing because when you use an LOD in an aggregate calc you can sum it to meet the aggregation requirements but it will still behave like a row level value.
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Both give the same results, just that you've already defined the aggregation within your first calculation.
The second one does it as a an aggregation by tableau.
That's why in the first one it says agg(sum-fixed-sum) in the green pill (as you've already aggregated it within the calculation).
The second one does sum(fix-sum) because tableau uses an aggregation type of sum by default.
Watch this video from Tableau zen master Andy
Read the comments at the bottom for further clarification from experts
Hope this will help.