As MBEW is supposed to contain current values, when it has to be changed (like when updating material current price, etc.), the current value gets written to MBEWH and the new value gets written to MBEW.
The current standard is always in MBEW. When the first activity in a ‘period’ takes place valuation or quantity update; a record is written to MBEWH to document the balance, std, price control, that existed BEFORE the "first activity.
The number that represents the previous period record being written into the MBEWH table is represented in the ‘current period’ LFMON field.