Placebo pills vs missing a pill?

Why are you protected on you placebo pills and considered unprotected if you accidentally miss a few pills? What’s the difference?

Update:

For context why I’m asking: I take Tarina Fe 1/20 which has 21 active and 7 placebo. My doctor recommended taking 3 extra active pills directly after my 3rd week and only 4 placebo pills (24/4) instead to see if it’ll help regulate my period. My march-april pack was my first time using this method so this is my second month. She said I was protected on those 4 days, and so I had sex (with a condom). I’m having PMS symptoms (2 weeks into my pack) which I’ve had before but it’s still anxiety inducing. I’m just curious why this method still protects me when missing 4 days mid-pack would be considered unprotected?

And if you were using pills to skip your period, say for 3 months perfectly with no placebos, does the same reasoning apply so that you’re still protected if you accidentally miss a few pills?