A fictional reason for Custer's Defeat?
If every western movie and television episode happened in the same fictional universe (despite all their contradictions) there would be a contributing factor for Custer's Last Stand that wasn't in real life.
in real history a cavalry regiment had 12 troops or companies and a total regulation strength of about 1,200 men, though the actual strength was usually much lower and the number of men available for duty even lower, usually about 40 to 60 per company or about 480 to 720 per regiment.
All 12 companies of the 7th US Cavalry were at the Little Bighorn. On the morning of June 25th 6 men and a sergeant from each company were assigned to handle the pack mules under Lt. Mathey, and Captain McDougall with B Troop was assigned to guard them. They fought on Reno Hill on June 25-26.
Major Reno with Troops A, G, & M fought in the valley and later on Reno Hill. Captain Benteen with Troops D, H, & K made a scout and later fought on Reno Hill. And Custer with Troops C, E, F, I, & L (there was no J Troop) fought miles away and was wiped out.
In the Cheyenne episode "Gold, Glory, and Custer: Requiem", 11 January 1960, Reno states that he had Companies A, G, & M, Benteen had H,D, & K, and Custer, C, E, F, I, & L, omitting Company B.
Each cavalry company has a fork-tailed flag called a guidon. The 1833-1862 pattern was red above with white letters "U.S." and white with the red company letter below. The 1862-1885 pattern had a version of the stars and stripes, and the pattern from 1885 to the present is red with the white regimental number above and white with the red company letter below.
Custer's Last Fight (1912, 1925) has titles stating which companies were in each detachment, and also shows the guidons of the various companies, using the 1885 design, which makes clear which companies were in each detachment.
In The Great Sioux Massacre (1965) a guidon with the letter "I" is seen in Custer's Last Stand, correctly putting I Company in Custer's detachment. But a guidon with "C" is seen in Reno's detachment instead of Custer's, and a guidon with "L" is seen in Benteen's detachment instead of Custer's, perhaps indicating Custer has a smaller detachment than in real life.
In The Plainsman (1936) a cavalry company of 48 men is besieged on an island in a river, and 30 men out of 48 are killed, others wounded, just a month or two before Custer's Last Stand. The guidon clearly says "B", and the number "7" can also be seen on it.
In Little Bighorn (1951) a troop of cavalry racing to the Little Bighorn to warn Custer he is badly out numbered (and thus obviously not part of his column) has a guidon with the letter "C" and no regimental number. Thus they are troop C of some cavalry regiment. If they are troop C of the Seventh Cavalry (which was in Custer's Last Stand in real history) Custer will have one less troop than in real history.
In Warpath (1951) Company M of the Seventh Cavalry, commanded by fictional Captain Gregson, suffers heavy casualties in a fight on a river island soon before the Little Bighorn. This fight, like the one in The Plainsman (1936) is obviously inspired by the Beecher Island Fight on September 17 to 19, 1868. Company M is also weakened before the Little Bighorn by a detachment detailed to escort a civilian wagon train through dangerous Sioux territory.
In Bugles in the Afternoon (1952) a troop of the Seventh Cavalry suffers heavy casualties - in a fight obviously inspired by the Wagon Box Fight on August 2, 1867 - soon before the Little Bighorn. This troop might be Company M also, since Captain Myles Moylan, who commanded Company M at the Little Bighorn in real life, is a character in the film.
A cavalry unit, Company D, from fictional Fort Starke is in The Command (1954) with a fictional date of 1876 or possibly 1878. A number of tribes have gone on the warpath after hearing about Custer's Last Stand. Second and seventh sound a lot alike, and I couldn't tell if they said 2nd Cavalry or 7th Cavalry.
So in any fictional universe that includes The Plainsman (1936), Little Bighorn (1951), Warpath (1951), Bugles in the Afternoon (1952), and The Command (1954) Companies B and M of the 7th cavalry, and possibly a third company, suffer heavy casualties soon before the Sioux campaign begins, while Companies C and D of the 7th might be detached for other duties and not with Custer's command.
And in The Great Sioux Massacre (1965) Custer's detachment could be smaller than in real history, since companies C and L might be with Reno and Benteen and the only company there is evidence is with Custer is I.
So any wild west movie universe including those movies offers a fictional explanation for Custer's defeat, because obviously Custer needed fewer cavalrymen than he had in real life about as much as he needed - as the old saying goes - more Indians.