FWIW, 5/8" is about five-thousandths of an inch smaller than 16mm.
I've never messed with wheels and axles (a man's got to know his limitations), but it seems to me that, except in the realm of precision instruments or space capsules, 0.00492" is within manufacturing tolerances of many nuts, bolts, threads, hex keys, etc. So my guess is that there was earlier -- by the supplier? at the plant? at some shop? -- manufacturing defect or cross-threading or other screw-up along the line. Put another way, I don't think it's an issue of which of those two sizes is used.
But I could be completely wrong, of course. Just ask my first wife.
2012 XCT (SOLD)
: HeliBars; Centramatics; PC-V; Lloydz air filter; 11" Madstad; Rivco LED mirrors; Doran TPMS; Stebel horn; Motolights; CustomLED trunk flasher; KewlHeel shifter; CycleOps trunk rack; BeadRider seat; Vibranator bar ends; Passenger grab handles; Powerlet outlets; Handlebar RAM mount; RicZ laydown plate; Brukus SaddlebagSecure; 14" flexible antenna; RAM cupholder on trunk; shock Schrader valve relocation.