Calculating Period over Period % variance

[43918084]

I have a formula to calculate the % Variance between t1 vs t0 using below formula

(if(isnull(sum(t1_amt_HKD000)),0,sum(t1_amt_HKD000))/if(isnull(sum(t0_amt_HKD000)),0,sum(t0_amt_HKD000)))-1

But the result does not come out correctly below.

I have also tried below and the result returns a lot of null value.    

(if(isnull(t1_amt_HKD000),0,t1_amt_HKD000)/if(isnull(t0_amt_HKD000),0,t0_amt_HKD000))-1

I have 2 questions on my case.

(1) How to fix the % var formula?

(2) What is the difference of having sum(t1_amount) - sum (t0_amount) vs  t1_amount - t0_amount ?  Why they will return different result?

Thank you very much again.