What about fuel? did you buy exactly the same gasoline? Even at the same station, they can get their fuel from a different supplier.
The more lean "mix" could only be incomplete combustion if all other factors are equal. The unburnt oxygen will make the O2 sensor read leaner than the mixture really is. As an oxygen sensor, it can't see the unburnt, fuel, just the available oxygen.
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1986 GXL ('87 4-port NA - Haltech E8, LS2 Coils. Defined Autoworks Headers, Dual 2.5" Exhaust (Dual Superflow, dBX mufflers)
1991 Coupe (KYB AGX Shocks, Eibach lowering springs, RB exhaust, Stock and Automatic)
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