It could be many possible reasons, whether it's something deliberate or an honest mistake, I can see it being either. Without any proof really it's all guessing!
Based on just personal experience, SAs can definitely hide things for their clients. I've asked for it done for myself / had SAs send me photos and ask me if I want one held (really I think they just hide it and I am normally told to go in to the store soon). It's not with an upfront payment and different than ordering it for you or place you on a waiting list. It happens if they get some new or hot items, or they know a client who is looking for it...I think this is probably not even allowed in some places. So it could be that they went and checked if it's OK to instead sell it to you as you are here and ready to pay. This is most likely I think if it's a deliberate thing.
But it could honestly just have been a mistake and nothing deliberate. Something in their system, they just got it, they forgot, ... etc.
In the end stay firm with what you want to buy, and really if they have it and can sell it to you they definitely will - that's the job! Obviously if they can sell you something more expensive or less popular I expect them to try, but if you know what you want and make them see that they will understand and act accordingly. I always understand if they make an effort though especially if they are under pressure for numbers. It might also depend on the store location and the type of clientele they have (it’s probably different strategies for say, a local LV vs one with oversea tourists). Congrats on your new goodies