When is a judgment of acquittal not a judgment of acquittal

In U.S. v. Alvarez, the Fourth Circuit in an opinion by Judge Niemeyer, joined by Judge Michael and Judge Motz, reversed the trial court's "Judgment of Acquittal" and remanding the case for a new trial, because the trial court did not say that it had decided that the government's evidence could not support a conviction (even though that may be what the trial court meant to say).