Basically there are two parts to the start menu.
The top part (above the seperator) shows programs you have permanently "PINNED" to the menu.
The bottom part shows programs you most frequently click in the "all programs" list, and it can change a lot.
You can disable the bottom part, so it only shows what you have pinned yourself. I have personally disabled this also, so i can customize the menu as i like, and not have it automatically list by frequency.