I would say that they averaged together the 90k and 160k to get 125k, 125k+300k=425k, then they just went rounding-crazy and called it 420,000 (maybe they rounded toward the even digits to make it 420,000 not 430,000).
“An engineer explained to us that hundreds of ear impressions were gathered in the name of research, and while each one obviously boasted its own unique shape and size, one single characteristic remained uniform across the board: the entrance into the ear canal is not a perfect circle, it's an oval.”
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I would say that they averaged together the 90k and 160k to get 125k, 125k+300k=425k, then they just went rounding-crazy and called it 420,000 (maybe they rounded toward the even digits to make it 420,000 not 430,000).