As 1x drivetrains have taken over and cassettes have gotten wider, that has forced riders to essentially cross chain at the extremes of the cassette. The angle of the chain then causes extra wear to the faces of the rollers which degrades the shifting over time.
Depending on conditions and riding style this can happen well before a chain has reached replacement thresholds for linear wear, commonly referred to as stretch. While chains that have worn out laterally won't cause undue wear to the rest of the drivetrain, shifting performance definitely suffers.
When SRAM introduced their Flattop chains, they included rollers that were 0.010″ (0.254mm) larger which is enough to skew the measurement on existing chain wear tools. How much? Abbey claims that when measuring five full links, a traditional chain tool would underreport wear by 0.2%.
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