I believe that Jeter = throwing something away (carelessly), and Lancer = throwing something targeted at some goal. For throwing away trash, use jeter. For throwing something like a spear, use lancer. (By the way: A spear translates in french as une lance. Probably not just a coincidence)
Watch out for Duo's drop-down hints. Given the sentence "La fille lance une chaussure", it is clear that "lance" is a verb. What verb? (lancer). Lancer = to throw. http://www.larousse.com/en/dictionaries/french-english/lancer/46054
There is a lot of overlap, but some slight differences, too. For details, consult your dictionary app:
It has to do with the way Duo processes your answer. If you make a gender error, e.g., put "un chaussure" (WRONG) instead of "une chaussure" (RIGHT), it recognizes the error. But instead of telling you the gender of the article is wrong, it will suggest a masculine noun, i.e., "un soulier". It doesn't help reinforce the idea of "chaussure". It tends to confuse the matter when it is simply a gender error. There is nothing we can do to fix that. Sorry.