Generate Lease Details Report: Liability , Amort Exp calculation

Hello,

I am trying to figure out below fields calculations for "Generate Lease Details Report". Can you help me to clarify?

Finance and Operating Lease            :Liability

Finance Lease(IFRS Operating also) :Amort Exp

I see that Liability is calculated as: Beginning Balance - US Gaap Amort Exp

    My question is why do we use US Gaap value of Amort Exp to calculate Liability?

Also I see that IFRS Amort Exp is calculated as: Beginning Balance(Rou or Liability which is same for my sample)  / Duration in months

Then it is depreciating Liability month by month with above formula's result.

Expectation of my client here is calculating it as Liability / Duration in months instead of Begining Balance / Duration in months