Hello @steemitteam and @steemitblog the easiest way to explain this is.
(Difference between master key and posting key).

If its accessing a home.

The posting key is the gate key.
It will allow you access the inside of the home that's the compound but it never help you access the house.

The active key can help you get access to the sitting room and the dinning room but never in accessing the bed room.

The master password as its even called master can help one access the compound it can help you access the sitting room and dinning just like the active and it can help you access the bedrooms.

Once one losses the master password he has lost it all.