why (Calif.) apricot & peach jam will not set?
Pectin needs acid to work. Fruit never has the same amount of acid. Not only do blackberries have much more acid than apricots, but two different cultivars of apricots and even two apricots of the same cultivar but grown in different conditions will have a large difference in the amount of acid. Ditto for lemons. 

Your fix proves that this is the reason - once you added more acid (in the form of lemon juice), the problem disappeared. So, the reason "why" is simply "you didn't have enough acid in your jam". 

The question of "why would the company publish a recipe" - it publishes a recipe which works on average, but it can't make a recipe which works for all fruit grown under all conditions. I can't tell you why your fruit is lower in acid than average (although the amount of sunny days during ripening is probably involved) but it seems that it is.