Cumulative Balance inconsistency

Hello Experts,

We have a problem with the cumulative balance of a GL account. Please help me to figure out the way system calculates this.

The problem is with a GL account belonging to a Company Code in Mexico. The account is in USD. The company code currency is PESO.

The accounts' cumulative balance does not reflect the correct valuated amount in PESO at different period (month) end. Most of the month end it shows the correct amount of balance which is the exact amount converted from USD amount to PESO in the Month end forex rate. But some period end, the PESO balance does not reflect the valuated amount that should have been after the valuation. Now the valuation differences are not posted to this account but to another balance sheet adjustment account. So, please explain how the PESO value is obtained by this account?

Thank you.