While understandable, the way the game presents this information is very confusing to general players. Fertility is not the same as fertility. Therein lies the confusion, and the only way I know they could fix it is to change the term "fertility" when referring to the stats/percentage.
An animal can be fertile or infertile. Fertile means it can produce offspring. Infertile means it can't produce offspring, no exceptions. This is only signified in the game by whether or not that red exclamation point appears by the animal's name.
In regards to the fertility stat, an animal can have 0%-100% fertility. 0% means it's more difficult for the animal to conceive and it will have less offspring. 100% fertility means it can conceive easily and has a better chance of multiple offspring.
Fertility does not affect fertility. If an animal is fertile, it's fertility stat is important. If an animal is infertile, it's fertility stat is irrelevant. All animals will have a fertility stat, fertile or not. It could be that way, but it should be more clear.
Perhaps they could rename the fertility stat "productivity" or something similar so people can make the distinction. That way they can know an animal is fertile but not very productive, or another animal has the potential to be productive but is infertile so will never have offspring.