Thank you for this delightful puzzle. There's indeed plenty to notice! I had to keep the numbers small since my child isn't fluent with subtraction yet. The first snake started with 15 and 11; the second - with 26 and 18. Then we conjectured that the reason for different outcomes was probably because the second snake had the first number with two tens instead of one. So our third snake started with 25 and 21. Which showed us that tens aren't the culprit. Then I suggested a snake that started with 18 and 15. Hmm, what's going on here? But at this point my kid got pretty tired (lots of arithmetic) and distracted. So we turned the snakes into necks of a 3-headed dragon. Tomorrow we'll try to feed the dragon some pairs of numbers so it doesn't explode (Question 4 for experts in your puzzle).
Thank you for the feedback. I love the 3-headed dragon!