Why do the first newsletters always get best metrics?

Hello,

I've divided my subscribers into 5 separate segments based on the remainder when dividing the length of their email address by 5. For each piece of content, I typically send the newsletter first to one segment, which takes about 30 minutes. Then, I send the same content to the second segment, also taking 30 minutes, followed by the third segment, and so on.

I've noticed that it's always the first newsletter which achieves the best metrics in terms of open and click rates.

Does anyone have an idea why this might be happening?

Thank you.