Number of lessons varies
I have noticed that my students have different numbers of lessons for the same skill. Is this based on their previous lessons? How is the number of lessons they have to complete determined?
The crown level they are on can have an impact on how many lessons they get. For example, there may be 7 lessons in a skill set to get from crown level 3 to crown level 4, but to get from crown level 4 to crown level 5 in that same skill set may require 10 lessons.
Also sometimes there are multiple versions of a course. Duolingo A/B tests changes before releasing them to everyone, so some of your students may be guinea pigs with some new features/lessons/vocabulary available to them. Duolingo will collect data and compare the new version to the old version to help them decide whether to roll out the changes to everyone.