So we used Built-with to search for Shopify stores as a keyword in Cla...

So we used Built-with to search for Shopify stores as a keyword in Clay via our API. Clay found some matches for them. However upon checking that same company's domain directly via Built-with Chrome Plug in we could not detect Shopify. Could someone please explain me why is that and what are we doing wrong here? How can we be sure that the stores are indeed using Shopify plugin?