A commentum (from comminiscor) is, according to the Elementary Latin Dictionary:
an invention, fabrication, pretence, fiction, falsehood
At some point, a commentarium (and, I presume, its cognate forms) came to mean something much more similar to English "commentary/comment," e.g. Aquinas's In Aristotelis De Anima Commentarium.
How (and when) did commentum adopt this much more neutral meaning?