Just check all the post of users that tell their Curve card gets rejected because of Curve NOT having 3DS. I have the same experience. Curve just does NOT support 3DS.
What you are experiencing is a site that supports 3DS, but it is optional, it does not require it. The site checks if your card supports 3DS, it realizes that your Curve card does not support it, but as the site does not require it, it decides to continue the transaction anyway.
More and more websites (outside the UK) do require 3DS supported cards, that is why it is such a shame Curve does still not have it. Instead of 3DS Curve decided to develop P2P functionality first.This is in my opinion a big mistake.