Yep, that’s one of the shortcuts we took to build the first version of Sit the Test more quickly. We’re actively working on making the entire app responsive, but right now it’s definitely a pain to use on a phone. Sorry about that!
The test actually contains 50 questions, and you get a random sampling of 25 every time you take it. So be prepared for a few different questions the next time you try it. 
If you haven’t already, give the “CSS Core (practice)” test a try. When you get your score, it offers to show you Detailed results, which tell you exactly which questions you got wrong (and what the right answer for each was). You can also resit this test as frequently as you like, so you can practice until you’ve mastered the material in the practice test, then take the “CSS Core” test (which covers the same material but with different questions).