The calculator is right, think about it this way the larger diameter tire would cover more distance in one rotation than a smaller tire. I wouldn't worry about it though in fact your speedometer will be more accurate with the larger tires.
What I would worry about is traction with the 225's in the back I don't know whether or not that would be a problem
2006 BMW 325i Titanium Silver
Mods- 19" AG M310's hypersilver, LCI xenons, drilled and slotted rotors, OEM rear spoiler, OEM exhaust tips, brushed black interior trim, Viper 5904 alarm + remote start
Audio upgrades- Alpine S920HD, JL XD600/6, Jehnert XE200, Morel Hybrid Ovation II, Audioque 2200D, 2 Alpine SWR 12d4's (forward facing sealed from trunk)