The 2008 models have some hardware changes, basically a trick alternator as part of the 'Efficient Dynamics' technology. The alternator only cuts in when needed, and in addition uses energy from braking to charge the battery.
I suspect that the CO2 reduction is mostly down to the hardware changes, although software changes may also play a part.
Even if it has reduced your emissions the DVLA won't care and will always tax you based on the original CO2 figure on the V5
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E92 335i M Sport, 6MT, Sparkling Graphite / Saddle Brown / Dark Grey Poplar, Bluetooth, 6FL, Hi-Fi Speaker System, Heated Seats