After controversies with batteries, Apple decided to offer their clients interesting discount. When journalists published information that Apple's phones deliberately slowed down when the battery was relatively old, the American company offered its customers a battery replacement for lower price and this action lasted throughout 2018. The discount was quite noticeable, as in most countries people could save around 50% of previous price. As you can guess, a lot of people benefited from this offer and it can be proved by statistics that appeared on the Internet.
During 2018, Apple exchanged approximately 11 million batteries in phones. Usually, firm replaces 1-2 million of batteries within 12 months, so difference is large. Certainly, a significant part of the people who have used services of Apple are customers who were encouraged to replace their battery by the promotion described above. In 2019 situation with amount of replaced batteries should return to normal state.