27.2 seat post, 28.6 clamp way too small

The published specs for my 2018 Gepida Alboin all say the seatpost is 27.2 mm, and this requires a 28.6 clamp. I ordered a clamp (replacing the quick-release) and it is too small. The clamp fits perfectly on the smaller seat post, but I'd have to spread it apart to force it onto the downtube. Any comments? Maybe the part itself is missized, or maybe the downtube is actually larger. I'll go down in a while to see if I can get a measurement, but this puzzles me.
I played this game with my Rad Power ebike and my E-Lux. If you ordered a strange brand clamp from Amazon its possible you got the wrong size with the correct number etched on it. I ended up buying my seat clamp from a BMX supplier. The seat clamp was around $20 and it fits perfect.

Measure the clamp you purchased with a set of calipers and insure that the inside diameter is correct, and also measure your frame just to verify.

And how tight is tight? I had to palm bump my RadRunner clamp on. I think the paint on the frame was on the thick side. Once on it works perfect.
Update--I went to the bike shop and they scrounged up a clamp that works. I never did get the size straight but I think it was a 30mm.. Anyhow, what should have been the simplest of tasks wasn't. There's a number of websites that confidently proclaim that 27.2 post = 28.6 clamp. There is no standard like that. But it's done. The new clamp is much stronger with two bolts. The QR was aluminum and it had been overtightened, pulling the bolt head through the hole.
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I had trouble with mine too. I replaced the quick release with a bolt .I'm tall so I don't have to remove my seat take out the battery.
Some clamp manufacturers, especially the off brands, specify the outer diameter of the seat post while others use the outer diameter of the seat tube. The trick is to know which is which. Often, the published description is vague on this point.