Agility was just substituted into the sentence, they didn't make individual sentences. For example it will same the same thing for any other skill except that skill will be there instead of agility.
It's very simple, really. Hardly a mistake at all.
I do, however, wonder why they couldn't have implemented a check for a vowel at the beginning of the skill's name and left it up to that to determine whether "a" or "an" was necessary.