Qlikview sort by expression

[Anonymous]

Hello guys am back again with a sort by expression issue

In continuation to the question i asked earlier in the link below

https://community.qlik.com/thread/177092

i have the expression below but my sort using the expression

sum({$<v_Volume_in_Litres,Month=,MonthYear={'$(v_MaxMonthYear_1)'},Year=,[Account Business Cust Type]={'CPT'}>}[PYTHON Value]) + sum({$<v_Volume_in_Litres,Month=,MonthYear={'$(v_MaxMonthYear_1)'},Year=,[Account Business Cust Type]={'FLAT'},[Transaction Product Group]={'Fuel'}>}[PYTHON Value])

is always taking the largest of first expression and adding to the second expression and total is sorted based on the final total. 

to illustrate further i have the example dataset attached.

i have the table with dimension limits set to top 20 and top 30 in 2 different screens. some of them marked in yellow are in 30 but not when i set to top 20. This is happening due to the value of CPT being lower but the overall total along with FLAT is higher.

can any one explain why this is happening ?