Getting external compiler bad elements on standard code

So, I wondered for a while why would you get bad elements when compiling the AX code using the external parallel compiler (AxBuild.exe) on a clean AX 2012 R3 with CU9.
The error list would be something like:

*** Badlist contains 3 elements.
*** Bad element: \\Classes\AssetBookBonusMethod_IN
*** Bad element: \\Classes\PCImportModel
*** Bad element: \\Classes\VendInvoicePostBatchCleanup
*** Compile log:

I always imagined that bad element for sure means an error, so, opening AX would recompile again the elements, but indeed (as it would be normal) no erroris thrown.

Then, I realized the obvious; if you would open up the log file (C:\Program Files\Microsoft Dynamics AX\60\Server\MicrosoftDynamicsAX\AxCompileAll.html) there are just some warnings on these elements, no errors.

So, bad elements can equal warnings, not necessarily errors.