I'm guessing the combined US EPA rating of the X1 is much better than the TT-S because the X1 probably has a super deep overdrive gear having it down below 2000rpm even at higher speeds, vs the TT-S probably having quite short performance oriented gearing putting the engine at 3000rpm and over most of the time. So not really a relevant comparison until two like vehicles with similar gearing are tested side-by-side. The Audi 2.0T engine is capable of some pretty impressive fuel mileage as well. To get these figures it's pretty much a guarantee that the BMW has super tall gearing, and if an automatic that it's mapped such that the engine is kept at as low of an RPM as possible at all times. Won't be able to have any fun unless you're in DS or manual mode, most likely.
2012 E70 X5d Deep Sea Blue / Sand Beige, Premium, 3rd Row, Multi-Contour
2011 E93 335i Deep Sea Blue / Oyster, Step, Premium, Convenience (PDC & CA)