I don’t doubt the fact that they take some margin to extend the lifetime of the battery, but if we take iPhones as an example, they:

• charge at a slower rate when nearing 100%
• try to postpone charging the final 20% until the last moment before disconnecting from the wall outlet
• can be software capped at 80% by the user (in newer models)

This makes me suspect that that the margin between what’s reported in software as 100% and the actual capacity of the battery is less than 20%. This also makes sense from the standpoint of the consumer expecting a long battery life on their expensive high-end device, putting pressure on the companies to make the margin smaller and the charging algorithms smarter. Just my observations, of course.