I ran into a similar problem when I tried to use a 350X front hub and rotor on a 250R front end. They look really similar and I would have bet money on them being interchangeable but they weren't. Maybe someone swapped the hub/rotor to an R one at some point.
John
1985 Eddie Sanders ATC330r
1986 Monstecate
1984 ATC70 (awaiting 125cc transplant)
1984 ATC125M
1986 GSXR 750 - Keepin' it old school!
1999 Arctic Cat 500 4x4 plow machine
1999 Arctic Cat Thundercat 1000 (lake lizard)
1996 Arctic Cat Jag 440 L/C (ditch banger)