Weird lesson lengths?
I've become accustomed to an average of seventeen questions or so if I don't miss any in a lesson. Tonight I did Spanish Determiners lesson 1 without errors but was given 24 questions, including a string of what seemed to be questions on un/una. Then the last lesson, #7 I think, in Ukrainian Letters 3 passed me with only 9 questions.
Duo calls them "adaptive" rather than "weird". It tries to figure out how many exercises you actually need. As far as I know, they haven't explained how it works, so I don't know if its guess is based on how much difficulty other users had with this lesson, how much you had with previous lessons, some combination of the two, or something else entirely.
Interesting, from the OP's description it would seem that they are attempting to help you practice more and strengthen prior skills without always requiring the use of the "Strengthen skills" button. Chances are that this is partly due to the new strengthen algorithm they implemented.