It's indeed more likely the police just bought an off-the-shelf measuring device. They often come with both imperial and proper units.
> If there was a conspiracy I think they would not have made such a foolish and obvious error.
Alas, that's not necessarily a good rule-of-thumb. Incompetence is a thing.
Compare to eg how the Russian spooks placed three copies of the game SIMS 3 in the pictures they published of an alleged assassins' lair.
(Most likely, some superior officer told the grunts to put three SIM cards for burner phones in the picture.)