not allowed to create calculation between two calculation that I created in designer

Dears.

I need dispaly accumulative sum value to analysize data, and my Bex datasource haven't this mearsure. I found I can create calculation(Accumulative Sum),and I have create succees,I create Accumulative1,Accumulative2. But my issue is that when I use the Accumulative1 to divided Accumulative2,the result isn't what I wanted.Is this a bug?

Follow picture shows my action.

Waiting for your help.

Thanks.

Jing