I've seen some 550 cord that is 4.5mm and some that is 4 mm? Both made in the usa. I was looking at Michael and joann craft stores for my small projects and it says 550 usa but compared to paracord galaxy there is the 4.5mm
You won't notice a difference in making projects. As a test, I recently took some fat, 8 strand mil-spec cord and fused it to the floppiest commercial cord I had. I then made a fishtail bracelet with a four-strand core. I thought for sure it would come out looking asymmetrical. Nope. Looked perfectly fine. I'd sell it to a paying customer in a heartbeat.