See
https://365.strat-o-matic.com/help/hittercard # -- Ballpark home run reading. An additional roll determines whether there is a home run or a flyout. Obviously, the probability varies by ballpark and whether the hitter is lefthanded or righthanded. The original result listed is ignored -- in effect, the result will be determined by "rolling against" the ballpark's home run rating (a random number from 1-20). If the number rolled is equal to or lower than the ballpark's rating for a left- or righthanded hitter, it is a home run.
This example is in the sample card shown:
#7- HR 1-17
fly(rf)B 18-20
The key is "The original result listed is ignored--"