I can’t imagine why they would do it as a random sampling. Makes no sense to ask someone they are sure of accepting or rejecting…It does make sense to ask someone on the border where they want more information to make a decision. So I would take it as a good sign that you are still in the running. However, I’m guessing that they can’t accept everyone they asked for a letter from, even if they are all amazing letters. So, it’s certainly not a guarantee.