^Yep. they say do not top off for a reason. Basically what happens is the extra fuel you are putting in from topping off is being forced into the EVAP system. Its bad for the vent valve but the really bad part is that it soaks the charcoal contained in the canister with fuel. The charcoal has a limited life on how much VAPOR (not fuel) it can absorb before it no longer works as intended. Once the charcoal is completely saturated your out nearly $400 for a new canister. They are designed to last the life of the car (150k or so) but if you regularly top off you can cut their life by over half.
The moral of the story: STOP TOPPING OFF YOUR TANK