The Physics thing happens because of the way it is studied. I probably have the number of modules wrong but this is the basic principle...
Single Science qualification is based on P(physics)1, B (biology)1, C(Chemistry)1 modules.
Single Science and Additional Science qualification: P1, P2, B1, B2, C1, C2 modules
Triple science - Physics GCSE P1, P2, P3
Biology GCSE - B1, B2, B3
Chemistry GCSE C1, C2, C3 modules.
So when students sit down to do the P1 exam, most people will not get an A, but most people sitting P1 who are doing it as part of triple science will have a good chance of getting an A, as they are the most competent (at Science) pupils). If you saw the bell curve for the whole of P1, it would look like bell curves for other subjects, but if you look at only the results of people who end up with a Physics GCSE it looks skewed towards everyone getting an A. That is because most people who sit physics modules don't end up with a Physics GCSE.
A science teacher could probably explain it in a less confusing manner.