Is HP = torque x rpm? Since the torque specs on both are similar(at least for 8AT), so maybe the difference is to taper the engine rev on 320i N20 at 5000rpm to throttle at 181hp, but allows 328i N20 to keep pushing to 6000rpm and 240hp. I guess we would see dyno charts soon enough on this. For now, 328i does appear to be a $2k-3k(expensive) factory tune versus 320i.
As tim330i noted, the bmwusa data can be incorrect too, or else it would really be odd that 320i N20 MT only get 184 lb-ft@1250-4500rpm ....