What are those "cheap" parts kits made of
There's a variety of sources.  This is, probably not the complete list, but it can give you an idea.


Factory overruns.  There are accidental overruns, deliberate overages, clandestine overruns.  The problem is that you don't know if the overrun was quality controlled or not.  In case of clandestine overrun, it's also not know if the manufacturer was following the process, or they were cutting corners.
Factory rejects.  When quality control decides that the tolerance is out of spec, the whole batch is rejected.  Suppose that the batch of 5% resistors falls outside 5%, but within some larger number.   These are still viable resistors, only less accurate.
So called "new from the old stock".  Company cleans out its inventory, which was procured through qualified channels, because they no longer use the part in their designs.  The quality of such parts is normal, although shelf life may still apply.


Except for the last type, the history of the parts is usually not known, when you buy the kit.

Don't use these bargain parts in life and death matters: airplanes, cars, weapons, medical devices.