* The school: Different schools have different marking schemes, teaching styles, and levels of difficulty in their prelims. Some schools use prelims as a rigorous assessment to prepare students for the higher pressure of the actual exam, while others design them as a more relaxed practice run.
* The subject: The relative difficulty of prelims versus the standard grade exam can vary significantly between subjects. A prelim in one subject might be significantly harder than the final exam, while the opposite might be true for another.
* The student: A student's individual strengths and weaknesses will also influence their perception of difficulty. A student who struggles with a particular topic might find the prelim harder, even if it's generally considered easier than the final exam, while a student who excels in that area might find the prelim much simpler.
* Exam board's marking scheme: Even with the same subject, differences between the marking schemes can influence the perceived difficulty.
In short, there's no universally applicable answer. Some students find prelims harder, some find them easier, and some find them about the same level of difficulty as the standard grade exam. Prelims are primarily intended as a practice and assessment tool, not necessarily a perfectly calibrated reflection of the standard grade exam's difficulty.