No surprise there. There are only four major lead-acid battery manufacturers in the USA, so they make them all regardless of label. By the way, Interstate isn't one of them - they are just another private label.
There are some additional specialty manufacturers, though, e.g. Trojan, Concorde (Lifeline), US Battery, etc. US Battery is a 6v GC specialist, Trojan for deep cycles & AGM, Lifeline for AGMs, etc. US Battery makes 6v's for quite a few private label brands.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL