Address Import ID is the address identifier that should stay with a particular address. Copy address to alternate is used when an address is being changed, the new address is added as the preferred address so this needs a new address import ID. But the current process actually gives a new import ID to the OLD address, so data gets messed up.

 This makes address cleaning and re-importing more difficult because an address (with the same address import ID) may have been changed in the interim. SO when you re-import the data you are overwriting new address data with the old address.