When will a judge grant a motion for a new trial on the grounds of insufficiency of the evidence?

Appellant correctly argues that the record contains contradictory evidence on this matter. However, the court stated in Dietrich v. Litton Industries, Inc. (1970) 12 Cal.App.3d 704, 717, 90 Cal.Rptr. 856, 864:

In passing upon a motion for a new trial made upon the ground of insufficiency of the evidence the trial judge is required to weigh the evidence; and in so doing he may disbelieve witnesses and draw inferences contrary to those supporting the verdict. When the motion is granted upon this ground the appellate court may reverse only when, as a matter of law, there is no substantial evidence to support a contrary judgment. (Mercer v. Perez, supra, at p. 112, 65 Cal.Rptr. 315, 436 P.2d 315.)