Questions about ratings

[LoneRhino]

I'm playing a fictional league with both automatically import real historical rookies and use random rookies from all eras enabled. In 1902 I acquired a 21 year old Dave Stieb. His overall rating and his potential were both 35 out of 80. This seemed low so I checked his pitching ratings in the editor and got this:



These numbers seemed low. IRL, by age 21, Stieb spent part of the year in the minors, and then started 18 games for the Blue Jays. I started a game with the 1979 Blue Jays and his editor page was this:



As you can see, they're radically different. Why is this?