Strange image transformation billing

We have certain amount of images and its quantity is not increasing. We've enabled image transformation for them with only option format=auto to get avif images for supporting browsers. We see this billing information. Everyday number of transformations increasing by ~30. It looks very strange since docs says: one transformation per month per image. It should be like all billable transformations should happen at the beginning of the month and then nothing for 30 days since number of images on the site is fixed.