IMO, it depends, maybe folks with more experience than me can chime in.
They must have felt that the other three batteries were in good shape, or this was the cheapest way to get rid of you for another year
One thing that I have consistently read is that when replacing just one battery you should do your best to match the new battery with the condition of the others for a balanced pack..... or maybe I was watching the dog whisperer when I heard that, I cannot remember...
I have replaced a single battery in my 3+ year old pack just a few months ago. That kept me from having to buy a new pack before I went home for summer, but the pack will need to be replaced in the fall as other batteries are weakening.