My old bike has a 23mm seat post, I found this product that says it's 23.4mm - that'll do I thought.
I went to my local Decathlon, found the item, clearly marked up and advertised as for bikes that take 23mm (certainly no mention of 21mm anywhere), It looks like a decently made product and I don't know if mine is defective? it definitely has 23.4mm printed on it. Anyway, I didn't have my vernier callipers on me at the time to check so it was only when I got home and tried to fit it that I realised it is way too thin.
I didn't intend to use an adapter but I tried both the sleeves that came with it, one was still too thin to tighten up the post enough to actually ride (the seat would still turn and no doubt sink if I sat on it), the other wouldn't fit into the frame with the post in it at all.
So if you are looking for a 23mm seat post this probably isn't the right product (even though it says it is)
Response of the brand
Hello. Thank you for your feedback which is very interesting. I have measured a couple of the seat posts currently in store to double check for any error and they measured up at 23.4mm as advertised. Please do return the seat post to your nearest store to be double checked and swapped for the right size if required.