Whether the alignment is spot on or not, you will wear the rear tires faster than front.
Same exact thing on my 09 328 - Conti DW staggered setup - bought (2) sets of rear tires to (1) set of fronts. Tires were worn evenly.
The fronts however, have a tendency to wear faster on the inside edges about 2" in before the rest of the tread is worn - due to alignment specs.
I have my car aligned by dealer 1X/yr and get Before/After spec printouts - always right in the required range for all settings.
I was told both by dealer & Tirerack that this is normal...
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