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Dynos only measure torque at each RPM and then compute HP at each RPM using the formula HP = Torque x RPM / 5,252. This means that HP and Torque lines will always cross at 5,252 RPM.
Your dyno results make sense. Though others have measured just under 400 HP at the wheels that's due to the variation of each dyno. You can do the computation yourself for each RPM to see if it computes out. In your diagram, you're not getting to multiply big torque numbers by big RPM to create numbers higher than 360 HP.
What you're reacting to is that the B58 is notoriously good with torque down low and throughout the midrange, which makes it more fun to drive around town. You'll notice that the torque starts declining around 4500 RPM. BMW rates the maximum torque for the B58B30O1 as a flat 369 LB/FT between 1500-4500 rpm, which your dyno reflects nicely (although it seems to show a bit more than 400 LB/FT down low). BMW notoriously under-rates its engines, and the B58 is a fairly extreme example. Dynos measure torque at the rear wheels while manufacturers measure and rate at the engine. A reasonable estimate for drivetrain loss between the engine and the wheels is somewhere between 15% and 20%. Conservatively, this means your car is generating over 420 HP and 500 ft/lbs of torque. Feel good about this!
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