And it's not uncommon for directors to cut together film so that it has nothing to do with what you're hearing.
Watch something like The Song Remains The Same, and see if you can count how many times Jimmy Page's fingers don't match up to what you're hearing (John Paul Jones fingers don't match up during the beginning of Stairway To Heaven either).
Now why something so obvious as a clip of Freddie not singing when you can HEAR him singing made it into the final cut, I dunno.
"Compromise? That word is not in our vocabulary"-Roger Taylor, quoting Freddie Mercury