I know it's very confusing and a lot of the sales associate are confused as well. The original flap bag was designed by Coco Chanel and then released in February 1955 - hence the name 2.55. The original looks exactly the same as the reissues. Over the next 50 years there would be updates to the original 2.55 such as changing the turn lock from the original mademoiselle closure to the CC closure, or changing the chain to the updated chain with the leather interwoven.
The 2.55 comes in 5 sizes (224, 225, 226, 227, 228) the "classic flap" comes in (east/west = small, medium, and jumbo). Although there are a few variations on the classic size as well (there have been tiny flaps).
Technically they are the same bag, one borne out of the other, but style wise they are different. So was that confusing enough?