There might be any combination of three reasons.
First, the Study Notes are summaries of the recommended readings itemized by the associated Learning Objectives which are themselves–according to GARP–the “detailed knowledge points” which support the “knowledge domains covered by the exam.” In several cases, answering the Learning Outcomes, as they are stated, requires assuming knowledge not covered exactly in the reading. The most common instance would be a reading (which, after all, tends to be a chapter in a book or part of the GARP books) that assumes some knowledge that may or may not be covered in the specific topic you are currently reading.
Second, and this is the most likely reason, our Practice Questions are written to avoid superficial coverage of the domains. With an intention to anticipate practitioner-oriented questions (the FRM’s actual stated methodological principle), our questions are more difficult than either average database-recycled questions or questions that might merely query summary points. Consequently, our questions commonly go into further detail than the Study Notes. But for most of our questions, this is because they are meant to help in preparation, not really to simulate the exact exam-style questions (which will tend to be slightly easier). In short: our practice questions are “deep dives” in order to give optimal practice, so they often contain assumptions (or steps, etc) that cannot be mapped to our Study Notes, which are summaries.
Third, the FRM syllabus cycles rapidly in general over time. As literal Learning Outcomes shift from year-to-year, certain concepts may drop out of the syllabus but they will, of course, remain in our question database. If they remain relevant, we tend to retain them in the Practice Question Sets. If they are not relevant, or less relevant, we try to relegate them to an Appendix. But due to the exam’s methodology, there is often not a strict “line of separation.” For example, some Learning Outcomes have been dropped one year but restored the next, with little likely impact on the exam.