Why will my MacBook Air never charge to 100%?

Please refer to If your Mac battery won’t charge completely for an explanation of "Optimized Battery Charging."

Optimized Battery Charging: If Optimized Battery Charging is enabled in Battery settings, the battery on your Mac laptop may delay charging past 80% in certain situations, even after the power adapter has been connected for an extended period of time. This behavior is normal and helps optimize the overall life of the battery. If you need to have your Mac fully charged sooner, click the battery icon in the menu bar, then click Charge to Full Now.

Further information is provided in About battery health management in Mac laptops, it depends upon your usage pattern. "Based on the measurements that it collects, battery health management may reduce your battery's maximum charge when in this mode. This happens as needed to ensure that your battery charges to a level that's optimized for your usage—reducing wear on the battery, and slowing its chemical aging."