Ok, so these aren't Lego brand parts, they are Chinese knock-offs. Historically, off-brand Lego clones haven't been very good. Lego always had very tight manufacturing tolerances other brands couldn't match, so Legos always fit together and come apart in a uniform fashion, while their copies frequently didn't.
I have no idea what the name of the company who makes this kit is, but I'm pretty happy with the fitment so far.
