Apper inconsistent with yum and dnf

[Peter G.]

Regularly, after having updated my F21 system with dnf, 
I notice that apper suddenly indicates that there are a 
few updates available. I rerun dnf and it says nothing 
to do. Then, I sometimes even try yum and it also 
indicates that no packages are available for update.

Why does apper constantly tell me there are updates 
available, even after I just upgraded? What 
repositories does it access that the others don't? For, 
when I click the install button in the apper plasmoid, 
something does appear to occur, although it is not 
possible to tell, from the lack of output, whether an 
upgrade is, in fact, enacted, but I would presume so.

What is happening?