Hello student, syllabus for jee mains and advanced are not variable. The questions you get in mains and advanced will definite be from the syllabus. The fixed syllabus is your 11th and 12th portions of all subjects. Maths, physics and chemistry. Syllabus is pretty much is that. But the questions and level of questions varies. If you take the toughest question in jee advanced, it has multiple concepts involved in it to solve. All the concepts would be from 11th or 12th. But combined. This way, the questions difficulty increases on the base  of fixed syllabus