I think there's a fair bit of development around 1, especially if they're on the move etc.
And with teeth, I think they notice it less when they're busy. So it really hits when they slow down for a nap or at night time, and stops them sleeping so well.
I also think that with so much of this we sit here and speculate, trying to work out what on earth is going on and how to sort it. Then 3 months or so down the line it all becomes clear (as you notice the tooth or the new skill or realise they were ill etc) and you wonder how you didn't realise at the time!.
All made worse by the bone crushing exhaustion. Which is almost worse when they have been sleeping because you know they can do it. At least A is consistent. Yet to sleep through at 13 months....