Why does my phone have two IMEI number and I have one SIM card?

imei slotI checked my IMEI number from the dialer by entering *#06# I can see only one IMEI number being displayed, the IMEI number of the second SIM slot is missing.If you have SIM card only in slot2 IMEI for that slot will be used otherwise if you have SIM card in slot1 or in both slots or no SIM card at