Yes, discarded and stale are two different things.
From the CGMiner 2.9 ReadMe...
SS is stale shares discarded (detected and not submitted so don't count as rejects)
DW is discarded work items (work from block no longer valid to work on)
[...] stratum may result in more discarded work but reduces rejected or stale shares so ultimately it is better.
Aaaah, thanks a lot! So my assumption would be that "discarded work items" have not actually been worked on, so they are nothing to worry about much - it's not lost computing time.
I hope someone can chime in and confirm whether or not that is correct.