Total difference in sumx and calculate function

Hello

I have written 2 measures:

qty 3 calculate = CALCULATE([total sales],FILTER('Sales table','Sales table'[QTY]=3)) 

THis gives total = 37404

sumx for qty equal to 3 = SUMX(FILTER('Sales table','Sales table'[QTY]=3),[total sales])

This gives total = 40236

But if i write the following fucntion the answer is right:

sumx for qty equal to 3 = SUMX(FILTER('Sales table','Sales table'[QTY]=3),[total sales])

Please help me to understand the logic, why sumx function gives different result when i write measure but when i write the formula it gives the correct answer.