There is another calculation that could perhaps come into play - even Gwede Mantashe has said that ANC MPs should never vote for an opposition-sponsored vote of No Confidence, arguing that they should resolve problems internally (essentially trashing those parts of the Constitution that provide for the removal of a President). The other side of this coin is that if there is a real fear of enough ANC MPs voting against Zuma anyway, then the ANC would prefer to do the job itself, in a controlled explosion, as it were.
Of course, this would involve some kind of "deal", which in the end would prove to be unsustainable in law.