The Discovery Centers are not owned or run by LEGO, they are part of Merlin Entertainment who also owns the theme parks, PaB there will vary.
The quality shall we say of PaB parts has a number of factors that affect what you find on the wall These factors include who's doing the ordering (or detail level of the order preparation), how fast the 'good' stuff sells once it arrives and even how much of the order was fulfilled by the warehouse(aka they don't always get everything they order). If you are going to one of the higher traffic stores such as the FL Disney store, the MoA store or the NYC stores, what you find for 'good' pieces can change rapidly. What is found for 'good' pieces can be very sensitive to trip timing, later in the day of the trailer delivery or the day following it often tends to be best. This gives them time to work thru the load and fill in the low or empty stock items. Late in the day on either weekend day tends to have the least desirable selection, sure they're filling the empty bins but it's likely they're running out of the 'good' parts or don't have a lot of time to see if they've got x, y or z already in the wall and here's a case of lovely red 2x4 bricks, filled!
Build? I merely Acquire atm.