Hi P . P . L ., it's because your rigs currently have 'trusted status' as to the reason why you're getting WU's to verify other peoples "problems".

Don't forget, these "problems" where your machine is verifying other peoples WU's are for a variety of reasons - some, yes, because their machine has errored out on a WU, others are to (re)verify that their machine is capable of producing valid results (which, by the way, other people occasionally receive WU's to verify that your rigs are also still producing valid results). There are also, those new to the project (either as new machines, or as completely new crunchers who've brought their machines to the grid), and their machines need checking/verifying as well.

The way I see it, is "money in the bank waiting to be cashed", and nothing I'll lose sleep over. Yes, it can be slightly annoying if you've got to wait a few days extra if the wingman hasn't returned their WU in the allotted time and thus, another copy has to go out on top of that, but in the grand scheme of things, if that's all you've got to worry about - then you've got your life pretty much in order.

Where is the problem?
You will be granted with the appropriate credit.
In case you do not know, no interest does apply to your WCG contribution
Cheers,
Yves