Are they keeping the other 50(in your case) for themselves?
Seems like if a certain broker gives full roundups out to multiple accounts belonging to one person, and TDA does not, then maybe some shares vanish into thin air? Or maybe they don't actually vanish, they just get put in a safe place.
Step aside son, you're blockin' the MOMO!
Disproving absence of evidence should be easy, show the evidence of.