People get this wrong because with many accents (most American regions among them) the "ed" in "biased" is almost silent when spoken aloud at conversational tempo.
I consider lip-movement during reading a mental disorder. It means your cerebral cortex is connected to your motor cortex in a flawed way. It's a significant 'brain wiring' issue, and would severely limit how fast one can read as well. It's not as bad as "mouth breathers" but close.
People get this wrong because with many accents (most American regions among them) the "ed" in "biased" is almost silent when spoken aloud at conversational tempo.