Students who are weak in chemistry do not read and remember. They may be spending time in trying to understand the system or practicing the problems. Chemistry requires reading, integrating, learning and revising. Students who spend time in reading and learning naturally do well in chemistry.
Physical chemistry requires thorough understanding of concepts to solve problems. Inorganic chemistry requires knowledge of properties and reactions. Organic chemistry requires understanding as well as knowledge of the compounds and their reactions. All in all, we find that there is a lot more to learn in chemistry than physics and maths combined.
Students who are weak in Physics, do not spend time in reading and understanding the concepts. They may be spending a lot of time in reading, remembering and practicing the problems.
Physics requires reading, thinking, visualizing, integrating and understanding. Students who spend time in reading and understanding concepts, do well in physics.
Students who are weak in math, typically, study without writing and practicing with pen and paper. They may be spending a lot of time in reading, remembering or trying to understand the problems.
Mathematics requires us to practice solving problems with pen and paper. Only by attempting problems in various ways do we become proficient in applying the quickest method in the shortest time. So, students who like to study by practising with pen and paper, automatically tend to excel in mathematics.