19th Century Injury Ratings

Just wondering about the logic used for injury ratings of 19th century players. Are they all for the most part grouped into the 1(not 1 Bold or 1 ital.) Injury rating? Is it just the thought they'd all get injured? I know of a handful of 0 I.R.'s, but no 1Bolds. I just had Cupid Childs get injured, for the max of 15 games. Never thought about it, but he played in 132 games of their 128 game record; with over 600 PA's (1896). Curious more than anoyed!

See Silent Mike Tiernan 1896. 0 I.R., with just north of 600 P.A. & 133 games played!