Thanks. I would say the yeast slurry had quite a bit of dead cells. So when you made your starter, you only saw the few remaining live cells perk up and show activity while the majority of dead cells were just along for the ride. So basically this is an under-pitch situation.
How do you change this for the better next time?
1) Put the old slurry in a container of 1.020 wort, shake it up and let it sit for 30min - 1 hour.
2) Have some new 1.040 wort ready to go, about 500 ml.
3) Pitch only the cloudy liquid part of the 1.020 container into the 500ml of 1.040 wort. (these are the active, live cells)
4) Step the 500ml up to 2-2.5 L of wort and so on to build up a pitch of fresh, live active cells.
Thems the rules and you have better outcomes when you follow the yeast rules rather than convenience which we ALL fall prey to. 1-2 weeks is the guideline for kept slurries. Anything beyond has too much cell death to be useful. Yes there might be exceptions but to do it right, that is the best way.