.575 is the "nominal" size but with repops, I've seen the bores going from .574 to .584 and everything in between. I've even seen originals at .579+. And yes, I shot an original 61 Springfield last weekend and it took .579 minies to be accurate. At 160+ years old, with issue sights and trigger, it'd still put 5 shots into 2in at 50yd all day. The bore size on the original doesn't surprise me one bit as it's not the only one I've seen larger than .575. BUT it was shooting my match ammunition for my 62 Colts.
The real problem here is there are far too many armchair types who believe everything they read about these guns and take it as gospel that with the marvel of replaceable parts, they'll all be the same. Problem is, we're dealing with wartime frenzied production by various companies, all of whom were trying to cash in on the arms race while being the lowest bidder. Couple that with the Europeans dumping their "old" stuff in the the American scuffle and you have a logistic nightmare with no real standard at all. The average person will pop on to utoob and believe what some reenactor says cuz "he's a Civil War guy" and then go to the range with his minies cast from wheel weight scrap sold by a round ball outfit, load his Civil War gun with .575 cuz "that's what they take", use modern Bore Butter cuz "that's what works in a T/C" and use lots of Pdex to roughly equal a 60g service charge and use questionable CCI blanks caps and then log into his favorite muzzleloading forum and whine about bad accuracy. This happens so often it's almost like a script.