Factual error: When Glass saves the Indian girl from the Frenchmen the flintlock pistol he gets off the Frenchman fires multiple times without reload, as it does later while being chased by the Indians.
Continuity mistake: When Bridger refuses payment for staying with Glass in the wilderness, his share of the money makes several mysterious movements on Captain Henry's desk throughout the scene. Although we see Bridger's share being handled only once, it goes from near the edge of the desk where Captain Henry placed it, to the edge of the desk in front of Bridger, to one moving bill in the middle of the desk, to all the money having disappeared from Henry's desk. (01:32:00)
Continuity mistake: If you look closely at Glass' face cut when Fitzgerald is biting his earpiece off, it is not bleeding when it should. In fact, it looks like the blood has been painted on his face.
Factual error: Stars in the sky are misplaced. The constellation Orion is on the wrong side of Pleiades and Taurus. In the movie Orion is placed above and to the right of the other two. In actuality, it is below and to the left of them in the sky. It appears that Orion was pasted into the frame, since the other constellations appear in their correct locations with respect to one another and the moon. (01:16:10)
Revealing mistake: When a force 10 gale is blowing and the CGI snow is horizontal... ... none of the trees move one bit, not one.
Visible crew/equipment: As Captain Henry rides off towards the end, there is a propane burner/heater on the ground to the left between the pine trees that he rides right past. (02:08:35)
Continuity mistake: When Glass and Fitzgerald are fighting at the end, watch closely as Fitzgerald pulls the knife out of Glass' knee after stabbing him and you will see the knife is soaked in blood. But it becomes clean seconds later before Glass stabs Fitzgerald in the stomach.
Continuity mistake: When Glass goes into the river to escape from the chasers, there is a bandage on his head. He loses it after the waterfall, then somehow the bandage comes back. When he manages to grab some wood to save himself, the bandage has gone again.
Continuity mistake: When he falls into water there is no snow around the river, but 10 seconds later there is some.
Deliberate mistake: At about 1:49 in the movie, Glass is awakened by an attacking force of Arikara, and he fires his single-shot, muzzle-loading flintlock pistol at the nearest one coming at him. Then he runs to his nearby horse and flees, turning back to fire the gun a second time immediately. No reloading and no second gun on his person. It never leaves his hand; he's whacking the horse's rump with it to goad it on as soon as he jumps on it. And then he fires that same gun another time in a matter of seconds. (01:40:00)
Answer: The French were lying to the natives. They had kidnapped the woman and then sold them provisions to help the search for her. They benefited further from the Native Americans killing their rivals.