You must follow the walkthrough exactly as written. Even if your project runs successfully and correctly, if the project does not meet what is expected from the walkthrough (the step-by-step guide; not the process documentation), then you can still fail.
One solution that exists in other websites, is to set a peer review assignment with one or more peers correcting it (and earning I dunno a badge or something?), it gives a community drive and would make way more sense than this poor auto correction.
Does this happen when you access the assignment result or any work item?
Try printing the transaction values when you run it and see if any value is incorrect. Or may be clear the cache and the least try reset your test data and run the process again.