I have attempted to buy the R mount adapter a few times when either this site or another reported that they were in stock at Canon USA. Using one link, I see that they are listed "in stock." Entering the site in a different way and clicking on the product tells me that they are out of stock. This tells me it is not just a question of being quick enough, but the website itself has some issues - perhaps duplicate pages that are relaying different information.
I think it’s still a question of being quick enough. Unless you believe Canon is just randomly listing items they don’t have as being in stock, which seems ridiculous.
There are product pages (with item descriptions, specs, etc.), and there is the online store. Many businesses, small and large, in-license an eCommerce platform to handle the transactional side. I agree that the two should sync up, but anyone who’s worked in a large corporation knows different platforms often don’t, despite complaints from customers and/or employees. Goodness knows I’ve complained about updates to information in one system not propagating to others many times over the years in large companies, with no effect. I’m in a smaller company and now at least when I complain about an issue like that it does get addressed swiftly (which I know has little to do with the size of the company but lots to do with the fact that now the request is coming from the C-suite).
I suspect that when they get something in stock, they manually update both the product page and the store. The eCommerce platform decrements the stock automatically and when it’s gone it shows backordered, but someone has to manually update the product page and that takes longer.
Also, as I mentioned above they don’t charge buyers’ credit cards until they are preparing to ship, so it’s likely they don’t actually subtract the item from the eCommerce stock level until they actually collect their money. If so, there’s a procedural reason for a lag period where something shows as in stock in the online store but all of the stock has actually been ordered (just not yet paid for). That is a much more likely explanation than some sort of malicious intent or even laziness, IMO.
FWIW, when I was looking for an RF 100-500 (but not convinced I’d buy one), I noticed that Amazon would show ‘in stock soon, order now’, then a couple of days later I’d see a Canon stock notice here or someone would mention availability on a thread, but when I would go to Canon USA it showed as In Stock on the product page but backordered in the cart. That pattern repeated a couple of times before I decided to order it (from Amazon as in stock soon; I had the lens in hand 10 days later). Meanwhile, people who had back orders with B&H were saying here they didn’t get one, suggesting their queue is long.