I am happy that you asked this question on the open podium. We all will know how the complex medium algorithm works.

I have some old stories that have earned only 3-4 till now despite having 1000+ reads. Then some stories have more earnings with just 10-20 reads.

Is it something to do with the length of the stories or something to do with what? No idea. As my 1000+ read stories were very long ones. At least 10 minutes read. So can it be that people read just 30 seconds of that 10-minute read story, hence the earnings is super low. In that case, your poems are usually short read time. If they have read means readers have read at least 30 seconds. And that is at least half the reading time. So your earnings don't look fair. Only Medium staff would know what's up with the Low EPR.