Should i use sum(sales) or LOD Fixed Customer Name Sum(SALES)?

HI,

A very basic doubt. I was making a simple bar chart, on Row shelf Customer Name and Column shelf sum(sales) and i got an output. Now i created {FIXED [CUSTOMERNAME] : SUM(SALES) } and i find there are variation between LOD Aggregation and normal sum(Sales) aggregation. Can you pls explain why this difference is there and also tell me which is the right expression to see total sales done by the Customer. Thanks in Advance