If you have following values -
The above table is virtual output of Aggr(Sum(Amount),Group) as well, if you surround it with one more SUM as SUM(Aggr(SUM(Amount), Group)), its output is 1000, means sum of all Amounts of virtual table you see above.
If you use SUM(Amount)/COUNT(DISTINCT Group) in text box then it will result into - 1000/3 (we have 3 distinct group)
If you use this expression in chart with Group as dimension then each row will have its Amount/1( each row has one count only)
So same expression show different results based on where exactly its being used, in text/image boxes or in the chart, different dimensions will show different results. Use of TOTAL inside expressions as well make difference as it indicates at which dimension level the aggregation is being performed.
Hope it helps!