How was that supposed to happen?
Wikipedia makes this clear.
David leads Calvin into his pod and launches into space as Miranda launches hers. One of the pods hits debris and is knocked off course. Calvin then lunges at David and attacks him as he struggles to send his pod into deep space, The earthbound pod performs a controlled re-entry and lands in the ocean (probably somewhere in the south pacific) near a boat with two fishermen. As they approach the pod and look inside, it is revealed to be David's...
Note it's doesn't say which pod is damaged & knocked off course.
This is a classic movie misdirect and cliff-hanger ending. We're supposed to think that the pod that landed on Earth is the "safe" one carrying Miranda.... but it's not and the threat isn't over