If eggs were laid by a hen, she went broody and sat on them for a few days (let's say four) and abandoned them they would die. Now if she laid eggs in nest for the same amount of days, then went broody and stayed the full 21 days they would hatch (assuming all goes well). I'm not sure if that answers your question. When a hen is broody she basically wants to/is hatching chicks. When a hen is broody she stops laying eggs, stays on the nest all of the time, etc.
She most likely started laying eggs in the other nest after you found the first egg. I thought it had been less time since you'd seen her, which is why I said she must have laid the eggs while weaning the chicks. Hens start sitting on eggs (going broody) because of a hormonal change. Summed up: she wants to and loves being a mom.
And the questions are totally normal! It's great that you are trying and willing to learn. Feel free to ask more questions any time.