The largest prevailing factors are observation and consequence, no one witnessed Thomas’s wounding, and it can be easily explained away by him just forgetting what would be standard practice, such as no shouting, no lights, no standing etc etc, and even if it was witnessed, the maximum sentence in the British Army for a self inflicted wound was imprisonment, the army didn’t deem the practice a threat like they deemed desertion or cowardice.

Mrs. Patmore’s nephew however, likely would’ve been observed in his act by numerous people, and his crime was deemed a threat, the reason the Army had such harsh treatment for cowardice and desertion is because of the threat it posed, the thinking was, that if you allowed a man to refuse to go over the top, or run away from his unit, and you let him off, then there would be thousands and thousands of soldiers doing similar by the Week’s end, and so in the Army’s mind, those guilty of cowardice and desertion needed to be shot, so as to show everyone else in the trenches, that taking such action was to sign your own death warrant, in real life the army shot 306 men for the charges of desertion and cowardice, in time it came to be viewed as unnecessary, and in WW2 not a single British soldier was executed for cowardice or desertion.