Egg hatched at the wrong level?

You must have picked up that egg when you were level 13.

My girlfriend and I had this happen too. My best guess is that 5km eggs are so common and the sorting system is so bad that really old 5km eggs end up at the bottom and you never opened them. Then with the Hallowe'en event you were walking more than usual and maybe not hitting as many pokestops and maybe using more incubators. I got down to 4 eggs at one point so it doesn't surprise me that I opened an old one and got a really low level Psyduck.

Now what I'm really curious about: are the eggs sorted in some specific order? Can you interpret what will hatch based on where the egg is? i.e. is it possible that the 5km pokemon I want but I've never hatched (e.g. Machop or Abra or Grimer, also want Growlithe but I've hatched one of him) could be just waiting in that bottom-right egg slot and I've never opened that egg?

Eggs are determined when you got them, not when you hatch them.

So if you saved a 10k from Level 10, until you were level 20 thinking you'd get a chance at a level 20 Snorlax, it would be a waste. You'd get a level 10 Snorlax