I don't get new lessons?
So my friend and I were studying Spanish together with Duolingo on our mobile devices (iPhone and iPad) and I noticed that some of his categories in the skill tree contain more lessons than mine. For example, the "Animals" section contains 4 lessons for him and only 3 for me, which means he has even more words to learn than I do. I'm pretty new here so maybe it's something I have yet to understand, but how is that possible?
You fell into two different test groups. Duolingo developers often test new methods on a limited group of students and compare their results with the results of the students in "standard" group. If the new approach is better, it is rolled out to everybody.