Chain wear. It’s something mentioned all too often in bicycle maintenance guides, but rarely correctly understood.

Bicycle chain wear explained - BikeRadar

chain checkers are not equal
"These are dependent on hand pressure to take a reading and, depending on how much pressure that is, they can be way off.

[Yes these gauges are affected by roller diameter variances, but I'd much rather replace a chain that's suspected of wear than replace an entire drivetrain later on.]

"The likes of KMC, SRAM, Park Tool and Abbey Bike Tools all agree that the 0.5 percent marking on most chain checkers is a safe point to replace the chain before cassette wear becomes an issue. You can push it out toward the 0.75 percent, but you risk overcooking the situation depending on the checker."

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"Remember, using a chain beyond its intended wear limit will prematurely wear out your cogs and chainrings so staying on top of this routine maintenance task can save you a lot of cost and hassle in the long run."
https://www.parktool.com/blog/repair...icle-section-5