sneakin explained it pretty well, but i dont really see what your worrried about. ever car is going to how your describing. unless you have a 2step your car is always going to see less vacuum at idle than at a steady higher rpm(in neutral with practically no load) mainly because if your throttle is only open 5% and your car is spinning at 4k it needs more air than at idle but that 5% throttle your giving it isnt giving the motor enough air to accelerate and the motor is pulling in so much more air because it revving higher that it actually starts creating a vacuum inside the manifold as it trys to "pull" in more air to sustain the rpm.

this strong vacuum also usually pulls the bov open because thats how a blow off valve works, it gets pulled open by the vacuum of the intake manifold under strong vacuum and/or deceleration. as soon as u stomp the pedal to the floor and vacuum slowly starts to turn into boost the blow of valve shuts because its now being pushed shut by the boost being fed onto the top of the bov spring from the intake manifold. and as soon as u let off the gas theres still pressure in the pipes and theres now vaccum in the manifold, both of these force the bov to open instantly.

kinda of confusing and its kind of hard to turn knowledge into words sometimes but i hope that helps