How does posting Key 70 calculate during ABAVN

Hi SAP Gurus,

Can you please tell me how does SAP tcode ABAVN calculates for posting key 70?  i had asset where when i post ABAVN, the total amount of posting key 70 is equivalent to the posted depreciation in my current year.

But the other asset different Chart of Dep, the posting Key 70 values are the total of posted depreciation from previous year + current year.  

What logic it picks up? it is because of the "Period Control"?

Please explain.

Thank you!

Eve