Luck is definitely a factor in interviews. You can never be fully prepared even if you 100% match the posted job requirements, so you better hope your interview relates to stuff you are prepared for....

This is, of course, just anecdote, but one of them was a good friend who failed their internship interview and passed a fulltime interview just 6 months later, both at Google... so there wasn't that much of a skill difference in-between.

sure, they have extra time to see you work before the conversion, but its still very expensive to host interns, especially ones you aren't confident in converting.