you would need to have taken all the 21 pills to remain protected in the 7 pill free days - I assume the hormone levels build up over the 21 days and then the levels drop a bit in the pill free days and thats why you get a withdrawal bleed. You dont get proper periods with the combined pill as it stops you ovulating so the bleed you get in between packs is due to the lowering hormone levels not because your body was preparing for a baby.
The combined pill works by switching the ovaries off and preventing ovulation. Roughly, speaking it takes 7 consecutive pills to turn them off and 7 consecutive pill-free days to wake them up.
It isn't true that all missed pills are risky. The 1st 7 in the pack should never be missed. Missing 1 or 2 of the middle 7 isn't a big deal as long as you take the rest of the pack correctly. Missing 1 of the last 7 isn't a big deal either as long as you then skip the pill-free week and run the next pack on.
However, its less confusing and more cautious to tell women to use precautions for 7 days after every missed pill!