I think it is basically if they want to sell it to you or not. Here is how my situation went down.
Wednesday, visit Hermes store:
ME: I would like to buy my wife a constance belt. It would be an intro to Hermes for us.
SA: We dont have any constance belt. We have the smaller H belts (32mm) The constance belt is bigger.
ME: I actually think that is what I want, I thought the all H belts were called the constant belts
SA brings out the belt kits, with different straps varying in sizes, from 95, 90, 85, and 2 different buckles in polished gold and polished silver. However, per the SA, she she only has the straps in the dark brown/black.
I pick the 90, and the silver buckle, and bring it home for my wife. However, it is slightly too long for her. Therefore, we need to exchange the strap, and my wife decides she wants to give me one too. So on Thursday, we visit the Hermes store. SA from Wednesday is not there. Here is how it goes:
Wife and Me: I would like to buy the H belt with a gold buckle.
SA2 : We dont have any of the H belts.
Me: I just bought my wife the H belt yesterday. The SA from yesterday showed me straps in varying size.
SA2: What I meant was we dont have any in men's size. I can put you on the waiting list.
Me to Wife in native language: I know they have it here, I saw the different sizes when I bought yours.
Me: Ok, I will be on the Waiting list.
Wife: Lets go to the LV store and pick out a belt out for you.
SA2: Can I get a measurement of your waist to know what size to order for you.
Me: Sure. (Measures waist, decides on a 95)
SA2: Actually, I think I have one in the back in your size. It is actually reserved for someone, but I can sell it to you now. However, all we have are these 2 gold buckles (not plain polished H buckles, but with designs that cost $60 more than the normal ones, even though I was shown the plain H buckles the day before)
Me: Sure. I will buy it. While I'm here too, I would like to exchange my wife's strap as the one I bought yesterday is too big.
SA2: We only have a strap in 85 that is a light brown and black, not the dark brown that you currently have.
Wife and Me: We actually like the light brown better.
By seeing how it went down on both days, they supposedly didnt have what we originally wanted, and tried to put me on a waiting list. When hearing that my wife wanted to go get a LV belt for me instead, somehow a strap magically appeared. It is also a bit flakey about what buckles/strap options they had.
You glow ! 
