What the shop is saying sounds right to me, though they probably knew that some time ago, like when they tried to order the bike. If they're new to the game, it's possible that they're as naive as you.
I have close association with the trade, so I know how it works. You have to order well in advance to get stock of these sort of bikes. You have to gamble on what will sell. The more you order, the better the price. Fashions change very quickly, and if you get it wrong, you get stuck with bikes that you have to sell at a massive discount, so no profit. If later your customers buy what you ordered, there's a high probability that you won't get in a second order in any one season. that means if you sold them all by June, you won't get any more until September, which will be next year's model.
It's a bit more complicated than that because say, as a dealer, you order 20 of a particular model. You can't put them all in your shop, so they're held at the warehouse and you call them off when needed. If you run out, there's still hundreds in the warehouse, but they all belong to other dealers. You ring up your sales rep to see if you can get some more. They then have to check with all the other dealers whether they don't mind losing any from their stock because maybe there's a chance that they're not not selling as many as they thought they would. Also, demand and fashions might be different in other countries, so they have to ring the German and French divisions to see whether they have unsold stock that they could transfer to UK. Basically, its a lot of phone calls and logistics with no certainty in anything. You can imagine that when they ring the dealers asking if they can have one of their stock, the dealer says that he needs time to think about it and probably doesn't bother to call back.
I have a similar problem to you. I ordered a new motorbike last week, which I should be collecting today, except I now find out that they can't get the colour I wasn't and the other colours are really horrible, like matt black or white and silver that looks like they forgot to paint the white. They told me that they had the one I want in stock when I ordered. I should have asked to see it. They have my deposit and can't give a date for when they can get one despite them being a main dealer and the bike being in continuous production without changes. Luckily, I'm in no hurry because I already have a nice motorbike.
There's a lesson to learn. If you can't actually touch the exact thing that you are buying, you should be suspicious about whether you can get it. If you can put your name on the one that's sitting on the showroom floor, you should be OK. Don't pay any deposit or make any other arrangement until you can touch the thing that you're ordering if you want to be certain.
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