Challenger was the 25th Space Shuttle launch. If you take 5 and subtract 2, like in Roman numerals, you get 3. If you add 10, because it was Challenger's 10th launch, you get 13. Spooky!
Columbia is even spookier because it was the 113th Space Shuttle mission. Or if you prefer, it's number was 107, which if you add the digits together you get 8. It was also the 27th flight of Columbia. If you subject 2 from 7, you get 5. Add 8 and 5 and you get 13. Spooky!