egg hatched, but she only stopped egg laying four days ago

I thought that it took eighteen days to hatch eggs. Maybe one was in the
nest box alone and I didn't know about it. Then perhaps she started laying
many days later with some time in between. It seems very odd.

One day I woke to find a cracked egg in my parakeet's cage. I had a male
with two females. The nest box came with the male and one female, but she
passed. So instead of three females and one male, I had two females in the
cage. I think they were fighting the day I found the cracked egg. I
separated the females and kept the male with the one that frequented the
nest box. He continued feeding her in the box. I noticed another egg in
the box. Then she began laying an egg every other day until there were
four. That was maybe twelve days ago at most. Perhaps an egg had been
hidden in the nest box in a corner. She doesn't keep them in the hollow
space much. I'm starting to wonder if she was even the one that layed this
egg that hatched tonight.

Either way, she's being a good mother to it.