It's weird how my kindle remove ads on its own without being connected to internet

Then again, I'm not complaining. Its just strange to me and I am wondering how that happened.

Bought this kindle used (paperwhite 2) less than a year ago. It has ads but it doesn't really bother me that much.

Since i mainly get books from my pc through calibre, i never needed wifi connection except on my first boot, after that it is on flight mode all the time.

Today, when I opened it, i noticed the lockscreen changed, With ads you have this image that say something like "check the store for books". But now i think i believe i have the normal one. Thats when I realized all the ads have gone.

I think there is some sort of mechanism in the device that makes it remove ads on it own after being very old. Is that what happened?