Poor health not noticed at the time of recruitment.might be another reason.
Google "Recruitment to the British Army During First World War by Alisa Stephens" and read items 10 and 17.
" 2 in every 5 volunteers (in 1915) were entirely unsuitable for military service.".
Perhaps a recruitment officer would approve a recruit without paying much attention and the medical officer also didn't pay attention to overall health and was only concerned with taking measurements. Only when the new soldier showed that he couldn't exert himself in basic training would the truth be revealed. (Just an idea).
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