Email Delivered to iPhone days later, deleted from Home PC?

I have noticed something lately.

I'll get email delivered by my ISP to my PC. I read/delete from my PC.

How come email that is sometimes a day older, sometimes even 5 days old will appear in my inbox on my iPhone even after it's been delivered and deleted on my PC?

I don't keep my phone's email application open at all. I tap on it, and sometimes there's a flood of emails.

Any ideas people?

Thanks in advance

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