All air in your house contains a certain amount of water vapor. The air easily holds this in the vapor state. But when air cools, the capacity of air to hold water vapor is diminished. You have probably heard of relative humidity. Air that hits a vapor barrier in front of insulation simply reflects back into the room without any change of phase (vapor to liquid). If that surface is cold however, the relative humidity suddenly increases (cold air cannot hold as much water) and condensation can occur.
The moisture that penetrates the wood ceiling is in equilibrium with the room and as long as insulation behind the ceiling is adequate, that equilibrium is maintained. It does not accumulate at the ceiling but diffuses back into the room. If you had no vapor barrier, the sir would continue to penetrate the insulation, and duing cold weather would surely reach the condensation point. As the insulation saturates, the condensation point moves closer to the room because the insulation looses air space to water. Eventually it starts to rain.
Thats about my best explanation. Sound like a meterologist yet?