It's more about getting you your purchase quicker than making Amazon more money. If you're buying an Oxo can opener, assuming no fakes, it's better for everyone for you to get the one shipped from 20 miles away than the one 2000 miles away because an Oxo can opener is an Oxo can opener. One should be just the same as any other. Only certain items qualify as well, nothing perishable, nothing with an expiration date (like batteries), nothing consumable (like shampoo), etc.
The seller can choose not to take part in that if they're worried about fakes being mixed in, they can have their specific items held apart from like items if they pay more. You can find out more about it searching on "stickerless commingled inventory"
Here's an old post (about Rubik's cubes) from when I was trying to figure out how it worked:
There doesn't seem to be a super easy way for the customer to tell if products are commingled, but I think if the ASINs (Amazon Standard Identification Numbers) don't match then the products are not commingled. Meaning the 356RS from the Gan Store is not from the same bin as the 356RS from CuberSpeed. There are also not multiple sellers listed for either item, which is probably another way to tell.
GAN Store - $17.99 ASIN: B07RSZ7W65 MODEL: 356RS https://www.amazon.com/GAN-Speed-Cube-Magic-Stickerless/dp/B07RSZ7W65
CuberSpeed - $16.99 ASIN: B01DJFVRLS MODEL: GAN356R https://www.amazon.com/CuberSpeed-stickerelss-Magic-Puzzle-Version/dp/B01DJFVRLS
Compare to something like Kerr canning lids, where the ASIN from different sellers is the same, so the products are commingled.
Customers First 24/7 ASIN: B00614S4NE Model: 00088 https://www.amazon.com/gp/product/B00614S4NE/ref=ox_sc_act_title_1?smid=ACBDL4ER0KTMZ&psc=1
ThingsOutlet ASIN: B00614S4NE Model: 00088 https://www.amazon.com/gp/product/B00614S4NE/ref=ox_sc_act_title_1?smid=A16FHJ3TYRLS8W&psc=1
And there you go. Assuming the GAN Store is actually GAN, it's probably safe as houses to order from them, and if you trust CuberSpeed, same deal. Also, if the ASINs match, probably best to always choose the cheapest price.