Does it mean that project comes to an end and only collect few pending WU ?
That's one possibility.
Does it mean the pool of WU runs low and need a refill ?
Many projects are designed to run continuously with a low pool. You'll never notice that a project needs a refill unless there's only one server that can supply your assignments because you'll just get WUs from another server. The pool is automatically refilled as soon as the WUs that are assigned for processing are returned. Having more WUs than are required by the science slows down the project so the ideal number is enough, but not too many.
Does it mean there is an issue with psummery ?
No. psummary is working as designed. It's generated automatically, and when a Server is down for longer that (some number) those projects disappear just like they would if the project happened to be finished. The server for p8900 has been offline. I'm not sure if any of it's projects are finished or not.
Is there a historical psummary where I can get k-factors from finished projects ?
Check the 3rd party forum. There's more than one source for that information. For example, the
bonus calculator fills in that information when you choose a project.