I don't think you get it. I can understand why.
The league only promotes two teams (three in League Two). It then offers the chance of promotion to one of the four teams finishing in the next four places in a new competition called... the play offs. Where you finish is irrelevant, you only get to enter if you are one of those four.
Once you can get your head around that it becomes clearer. Continually linking it to finishing 3rd, 4th etc is just bollocks. It is a separate competition like entry places for the Europa League. It took me some time, because if you are old enough you remember the old system and think like that. You shouldn't. Think top two = promotion, 3,4,5 and 6 = entry to a knock out cup competition where winners get promoted. Simples.

Finishing 3rd, 4th , 5th, 6th = not good enough to finish in the auto promotion places.